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 the answer is hidden within the clue.
'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" .)