Pants Penny’s wearing droop when pulled up (5)
I believe the answer is:
gasps
'pants' is the definition.
('gasp' can be a synonym of 'pant')
'penny's wearing droop when pulled up' is the wordplay.
'penny's' becomes 'ps'.
'wearing' is an insertion indicator.
'droop' becomes 'sag' ('sag' can be a synonym of 'droop').
'when pulled up' says the letters should be written backwards.
'ps' placed inside 'sag' is 'spsag'.
'spsag' back-to-front is 'GASPS'.
(Other definitions for gasps that I've seen before include "'Pants, puffs (5)'" , "Draws breath sharply" .)