Play squash or tennis keeping trim (7)
I believe the answer is:
shorten
'trim' is the definition.
(shortening is a kind of trimming)
'play squash or tennis keeping' is the wordplay.
'keeping' says the answer is hidden in the clue.
'SHORTEN' is hidden in the letters of 'play squash or tennis'.
(Other definitions for shorten that I've seen before include "Make less long" , "Prune" , "Put extra fat" , "Truncate" , "Reduce in length" .)