Nurtured as British left-winger (4)
I believe the answer is:
bred
'nurtured' is the definition.
(I've seen this before)
'british left-winger' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'left-winger' becomes 'red' (red is a kind of left-winger).
'b'+'red'='BRED'
'as' acts as a link.
(Other definitions for bred that I've seen before include "Very (music)" , "Generated" , "Developed (animal variety)" , "Brought forth" , "(Well) brought up" .)