Girl left bitterness (4)
I believe the answer is:
gall
'bitterness' is the definition.
(I know that bitterness can be written as gall)
'girl left' is the wordplay.
'girl' becomes 'gal' (synonyms).
'left' becomes 'l' (common abbreviation).
'gal'+'l'='GALL'
(Other definitions for gall that I've seen before include "A sore spot or a bodily bladder" , "Irritate - presumption" , "Audacity" , "Impudence - annoy" , "Bitterness of feeling or bodily bladder" .)