Girl left behind in America (4)
I believe the answer is:
lass
'girl' is the definition.
(lass can informally mean a girl)
'left behind in america' is the wordplay.
'left' becomes 'l' (common abbreviation).
'behind in america' becomes 'ass' (US term for one's behind).
'l'+'ass'='LASS'
(Other definitions for lass that I've seen before include "Young woman, girl" , "Damsel" , "Sweet resident of Richmond Hill?" , "Young woman (-. Eng.)" , "Scottish girl" .)