Instigate frolic in gym (6)
I believe the answer is:
prompt
'instigate' is the definition.
('prompt' can be a synonym of 'instigate')
'frolic in gym' is the wordplay.
'frolic' becomes 'romp' ('romp' can be a synonym of 'frolic').
'in' means one lot of letters goes inside another.
'gym' becomes 'pt'.
'romp' inserted within 'pt' is 'PROMPT'.
(Other definitions for prompt that I've seen before include "Ready" , "Help forgetful actor with lines" , "Instantaneous" , "Give cue to forgetful actor" , "Cause, bring about" .)