Fool girl, losing pound (3)
I believe the answer is:
ass
'fool' is the definition.
(ass is a kind of fool)
I can't explain the rest of the clue.
(Other definitions for ass that I've seen before include "Horse-type animal" , "Silly fellow in passionate state" , "Transport for a wise man - or a fool" , "Wally" , "An American backside" .)