Give up gym, stripping to exercise topless (5)
I believe the answer is:
yield
'give up' is the definition.
(I've seen this in another clue)
'gym stripping to exercise topless' is the wordplay.
'stripping' means to look at the middle letters (outer letters strip off).
'to exercise' becomes 'wield' (both can mean to employ or make use of).
'topless' means to remove the first letter.
The centre of 'gym' is 'y'.
'wield' with its first letter taken away is 'ield'.
'y'+'ield'='YIELD'
(Other definitions for yield that I've seen before include "return of financial investment" , "Render" , "Abandon" , "Concede, give in" , "Give way; produce" .)