Western girl left in bitterness (4)
I believe the answer is:
gall
'bitterness' is the definition.
(synonyms)
'western girl left in' is the wordplay.
'western girl' becomes 'gal' (synonyms. I am not sure about the 'western' bit.).
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'gal' enclosing 'l' is 'GALL'.
Can you help me to learn more?
(Other definitions for gall that I've seen before include "Sheer impudence from bladder?" , "This bladder is attached to the liver and stores bile" , "Spleen or brass neck" , "Effrontery" , "Rancour; a fungal growth" .)