Underwear finally dropped, producing gasp (4)
I believe the answer is:
pant
'gasp' is the definition.
(I know that gasp can be written as pant)
'underwear finally dropped' is the wordplay.
'underwear' becomes 'pants' (underpants).
'finally dropped' means to remove the last letter.
'pants' with its last letter taken away is 'PANT'.
'producing' acts as a link.
(Other definitions for pant that I've seen before include "Sound out of breath" , "Breath noisily from exertion" , "Gasp for breath (like a hot dog?)" , "Breathe hard" , "Long" .)