Hello there fellow DMs. Looking for some ideas.
An upcoming session is going to include an Underground fight that one of my player's has gotten into. I'm thinking I'm going to run the whole thing as more of an RP scene than a Combat. The Barbarian is the one that will actually be doing the "fighting" while their Cleric and Sorcerer are just attending and betting on the Barbarian.

What I'm looking for is ideas for things to do with the Cleric and Sorcerer during the scene, in order to keep them active, while the fights are going on.

A rumor gets out that some one has rigged the fight maybe a spell caster casting buff spells. They need to find who in the crowd is rigging the game and keep it unseen enough that the fight isn’t canceled.

maybe the ring has a rule that if a game is canceled all the money is placed in the pot for the next fight.

I did something similar once. I had assassins in the crowd that those, not participating in the arena, had to deal with. The assassins may be to do with some other group not involved or maybe the party has stumbled across something happening in the arena.

You could also give them an opportunity to rig the match a bit. It really depends on what kind of fight you want. Is it no rules where you have to win no matter what or is it a moment for the barbarian to shine?

Maybe there is someone important for the other PCs to meet. They could try and make contacts or perhaps assassinate someone themselves.

Perhaps there is a way of making a game around the betting. Something to do with cards and bluffing with the goal posts changing depending on what you have in your hand but this would require some thought.

I hope this helps.

Keeping the fight fair by stopping meddlers or being meddlers themselves can be a lot of fun.

You could also try to rig up a very simple system around hyping the crowd. For example you could have 5 possible positions from -2 to +2, where -2 is the crowd heavily favoring the bad guy and +2 has them heavily favoring the barbarian. Assuming the barbarian is the challenger, things might start at -1 and the cleric and sorcerer can do skill checks or complete other challenges to shift the crowd in the barbarian's favor (or perhaps against if they fail).

You could translate this favor into a bonus to attack and damage for the favored fighter, or have it affect the betting somehow, or whatever else might be relevant to the story.

There are pretty good rules for pit fighting and gambling in Xantathar's. I like scatterbrained's  idea of the 'bystander" characters hustling the audience.  Of course, in most underground fighting establishments, the house is profiting off of gambling proceeds too so may take an unkind eye to hustling (as one would in a pool hall where someone's hustling patrons, or more like a casino where someone's counting cards).  

Quote from scatterbraind >>

Keeping the fight fair by stopping meddlers or being meddlers themselves can be a lot of fun.

You could also try to rig up a very simple system around hyping the crowd. For example you could have 5 possible positions from -2 to +2, where -2 is the crowd heavily favoring the bad guy and +2 has them heavily favoring the barbarian. Assuming the barbarian is the challenger, things might start at -1 and the cleric and sorcerer can do skill checks or complete other challenges to shift the crowd in the barbarian's favor (or perhaps against if they fail).

You could translate this favor into a bonus to attack and damage for the favored fighter, or have it affect the betting somehow, or whatever else might be relevant to the story.

 It might also change the betting rates.

Some really good ideas in here. Its def given me some things to play off and helps! Thanks!

I'll write up my plans a little later today and post them here for anyone interested in them and maybe to discuss them some if you want.

For a sense of urgency, use homebrew buffing spells (perhaps through some tattoos on the fighter) to make them effectively god-mode whilst the sorcerer in the crowd is doing his thing. Make it so that if they don't sort the sorcerer out, the barbarian is going to get knocked out or even killed.

Alternatively, you could pick out a monster who would ruin the day of the barbarian in terms of a slugfest, and have the enemy fighter be that - but disguised using an advanced illusion spell by the sorcerer, who needs to maintain concentration. As soon as concentration breaks, the fighter is revealed as an Ogre or a Troll or something and the patrons will flee the place. The fight might then devolve into an all-out combat between the players and the monster & sorcerer.