Go into gym to become trim (5)
I believe the answer is:
prune
'trim' is the definition.
(synonyms)
'go into gym' is the wordplay.
'go' becomes 'run' (synonyms).
'into' means one lot of letters goes inside another.
'gym' becomes 'pe' ().
'run' going into 'pe' is 'PRUNE'.
'to become' is the link.
(Other definitions for prune that I've seen before include "Dull person" , "Shrivelled fruit" , "Trim - dried fruit" , "Cut back the dried plum" , "A fruit; to cut back" .)