Bitter gain with Left ousting Right (9)
I believe the answer is:
inclement
'bitter' is the definition.
(thesaurus)
'gain with left ousting right' is the wordplay.
'gain' becomes 'win' (I've seen this before).
'with' becomes 'w' (abbreviation).
'left' is a deletion indicator.
'ousting right' becomes 'clement' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'win' with 'w' taken away is 'in'.
'in'+'clement'='INCLEMENT'
(Other definitions for inclement that I've seen before include "Such harsh weather" , "rather wet" , "(Of weather) cold or wet" , "severe" , "bitter" .)