Crack's seen around top of short pants (5)
I believe the answer is:
gasps
'pants' is the definition.
('gasp' can be a synonym of 'pant')
'crack's seen around top of short' is the wordplay.
'crack's' becomes 'gaps' ('gap' can be a synonym of 'crack').
'seen around' means one lot of letters goes inside another.
'top of' says to take the initial letters.
The first letter of 'short' is 's'.
'gaps' enclosing 's' is 'GASPS'.
(Other definitions for gasps that I've seen before include "'Pants, puffs (5)'" , "Draws breath sharply" .)