Long or short trousers (4)
I believe the answer is:
pant
'long' is the definition.
(I've seen this before)
'short trousers' is the wordplay.
'short' means to remove the last letter.
'trousers' becomes 'pants' ('pants' can be a synonym of 'trousers').
'pants' with its final letter taken off is 'PANT'.
'or' acts as a link.
(Other definitions for pant that I've seen before include "Breathe hard" , "Breath rapidly" , "Breath noisily from exertion" , "Gasp for breath (like a hot dog?)" , "Sound out of breath" .)