Right-wing British left treaty partners taken aback (4)
I believe the answer is:
blue
'right' is the definition.
(colour of the right-wing Conservative Party)
'wing british left treaty partners taken aback' is the wordplay.
'wing' becomes 'l'.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'left' is a reversal indicator.
'treaty partners' becomes 'eu' (I can't justify this - if you can you should believe this answer much more).
'taken aback' shows that the letters should be reversed in order.
'l'+'b'='lb'
'lb' reversed gives 'bl'.
'eu' in reverse letter order is 'ue'.
'bl'+'ue'='BLUE'
Can you help me to learn more?
(Other definitions for blue that I've seen before include "Morose; colour" , "Colour; depressed" , "over-extravagant" , "gloomy" , "e.g. Indigo" .)