Spaces occupied by small pants (5)
I believe the answer is:
gasps
'pants' is the definition.
('gasp' can be a synonym of 'pant')
'spaces occupied by small' is the wordplay.
'spaces' becomes 'gaps' (gap is a kind of space).
'occupied by' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'gaps' enclosing 's' is 'GASPS'.
(Other definitions for gasps that I've seen before include "'Pants, puffs (5)'" , "Draws breath sharply" .)