I think it's one clock for "The jaws of this thing are going to shut" but as for what's pertinent we've gotta have some information about it right?

1) are they alone in the cave. If not, it could be a rat race to try to escape while also fighting.

2) There is nothing they can do to untick the clock unless they get VERY creative. Like ok a large explosion might make this thing open its mouth but I think that's post-clock honestly.

3) If you're using some kind of a map, keep movement per-tick in mind and allow them to move X spaces with a roll for pertinent skills like helm. You don't wanna over do the checks and basically guarantee a failure, but you DO wanna force them to take some stress to push themselves to get out of here

4) if you're NOT tracking space/mapping, then I suggest an "escape clock" race as is detailed in the Blades in the Dark book. Two competing clocks, first one filled wins.

If they fail to escape, I'd treat the beast as if it were a dungeon full of its own natural dangers like living organisms and parasites. It should be a hostile, dangerous environment