Girls going topless - the fools! (5)
I believe the answer is:
'the fools' is the definition.
(ass is a kind of fool)
'girls going topless' is the wordplay.
'girls' becomes 'lasses' (lass is a kind of girl).
'going topless' means to remove the first letter.
'lasses' with its initial letter taken away is 'ASSES'.
(Other definitions for asses that I've seen before include "Hardy and sure-footed animals" , "Pompous fools" , "Donkeys" , "Fools; beasts of burden" , "not the most sensible types" .)