Snakes behind girl’s pants (5)
I believe the answer is:
gasps
'pants' is the definition.
('gasp' can be a synonym of 'pant')
'snakes behind girl's' is the wordplay.
'snakes' becomes 'asps' (asp is a kind of snake).
'behind' is a charade indicator (letters next to each other).
'girl' becomes 'g'.
'asps' after 'g' is 'GASPS'.
(Other definitions for gasps that I've seen before include "Draws breath sharply" , "'Pants, puffs (5)'" .)