Jab acceptable in gym (4)
I believe the answer is:
poke
'jab' is the definition.
(synonyms)
'acceptable in gym' is the wordplay.
'acceptable' becomes 'ok' (OK can mean acceptable or fine).
'in' indicates putting letters inside.
'gym' becomes 'pe' (short for Physical Education).
'ok' inserted inside 'pe' is 'POKE'.
(Other definitions for poke that I've seen before include "Probe with stick" , "Jab or prod" , "Jab (with a finger)" , "Nudge; a bonnet" , "Ridicule - . . . . fun at" .)