Girl left by fool (4)
I believe the answer is:
lass
'girl' is the definition.
(lass can informally mean a girl)
'left by fool' is the wordplay.
'left by' becomes 'l' (I've seen this in other clues).
'fool' becomes 'ass' (both can mean a stupid person).
'l'+'ass'='LASS'
(Other definitions for lass that I've seen before include "Scottish girl" , "Young woman (-. Eng.)" , "Sweetheart" , "Young woman, girl" , "Sweet resident of Richmond Hill?" .)