Girl going topless, the fool (3)
I believe the answer is:
ass
'the fool' is the definition.
(both can mean a stupid person)
'girl going topless' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'going topless' means to remove the first letter.
'lass' with its first letter taken off is 'ASS'.
(Other definitions for ass that I've seen before include "Jennet" , "Hardy and sure-footed animal" , "Quadruped; fool" , "Titania loved one" , "Horse relative" .)