Does bum express pong escaping trousers? (7)
I believe the answer is:
sponges
'does bum' is the definition.
('sponge' can be a synonym of 'bum')
'express pong escaping trousers?' is the wordplay.
'trousers?' indicates a hidden word (trouser can mean to steal or take).
'SPONGES' can be found hidden inside 'express pong escaping'.
(Other definitions for sponges that I've seen before include "Cadges like sea animal" , "Primitive, porous, marine animals" , "Cakes" , "Light, highly absorbent skeletons of marine animals" , "heavy drinkers" .)