Nervous characters in crowded gym (4)
I believe the answer is:
edgy
'nervous' is the definition.
(synonyms)
'characters in crowded gym' is the wordplay.
'characters in' indicates a hidden word.
'EDGY' is hidden in the letters of 'crowded gym'.
(Other definitions for edgy that I've seen before include "Keyed up" , "Extremely" , "Sharp in outline" , "Jittery" , "Nervous; jumpy" .)