Irritable in overheated gym (4)
I believe the answer is:
edgy
'irritable' is the definition.
(edgy or on edge can mean irritable)
'in overheated gym' is the wordplay.
'in' indicates a hidden word.
'EDGY' is hidden in the letters of 'overheated gym'.
(Other definitions for edgy that I've seen before include "Apprehensive" , "Sharp in outline" , "Red shade" , "Jittery" , "Nervous; jumpy" .)