ST cryptic 2418

[zacharias]

30a Sentry that made everybody laugh?

Is it "comic relief" and if so, why?

[Mamyalynne]

could it be light relief??

[Christiana]

I have comic relief for the answer but consider it a very weak clue. I suppose it's because a sentry is on a watch and then gets relieved when another sentry comes on duty, but I don't like the answer at all.

[zacharias]

Thank you christiana & mamyalynne. I thought I might be missing the obvious. Not a good clue!!