Forsaken by a girl (5)
I believe the answer is:
lorna
'girl' is the definition.
'forsaken by a' is the wordplay.
'forsaken' becomes 'lorn' (similar in meaning).
'by' means one lot of letters go next to another.
'lorn'+'a'='LORNA'
(Other definitions for lorna that I've seen before include "-- Doone, novel" , "Female" , "girl" , "and 21 Down: Novel by R.D. Blackmore" , "-- Doone (book)" .)