Long underwear to be cut short (4)
I believe the answer is:
pant
'long' is the definition.
(I've seen this before)
'underwear to be cut short' is the wordplay.
'underwear' becomes 'pants' (I've seen this before).
'to be cut short' means to remove the last letter.
'pants' with its last letter taken off is 'PANT'.
(Other definitions for pant that I've seen before include "Breathe heavily" , "Sound out of breath" , "Breathe noisily after running" , "Long" , "Struggle for breath" .)