Pocket in short pants (7)
I believe the answer is:
trouser
'pocket' is the definition.
(I've seen this before)
'short pants' is the wordplay.
'short' means to remove the last letter.
'pants' becomes 'trousers' ('trousers' can be a synonym of 'pants').
'trousers' with its last letter taken off is 'TROUSER'.
'in' acts as a link.
(Other definitions for trouser that I've seen before include "Pocket (something)" , "Pocket (slang)" , "For oneself" , "Our rest (anag)" , "Appropriate" .)