Play in short trousers (5)
I believe the answer is:
slack
'play' is the definition.
(slack is a kind of play)
'short trousers' is the wordplay.
'short' means to remove the last letter.
'trousers' becomes 'slacks' (slacks is a kind of trousers).
'slacks' with its last letter taken off is 'SLACK'.
'in' is the link.
(Other definitions for slack that I've seen before include "Lax" , "Loose" , "Flaccid" , "coal dust" , "Limp - negligent" .)