Girl losing head, the fool (3)
I believe the answer is:
ass
'the fool' is the definition.
(both can mean a stupid person)
'girl losing head' is the wordplay.
'girl' becomes 'lass' (lass can informally mean a girl).
'losing head' means to remove the first letter.
'lass' with its initial letter taken off is 'ASS'.
(Other definitions for ass that I've seen before include "Titania loved one" , "animal inside" , "Horse-type animal" , "Animal related to horses" , "oaf" .)