Sir Nigel turned white (8)
I believe the answer is:
riesling
'white' is the definition.
(type of white wine)
'sir nigel turned' is the wordplay.
'turned' indicates anagramming the letters.
'sir'+'nigel'='sirnigel'
'sirnigel' anagrammed gives 'RIESLING'.
(Other definitions for riesling that I've seen before include "Sir Ingle makes wine from this grape" , "Aromatic white wine from the Rhine valley" , "Fragrant white wine from the Rhine valley or from California" , "German/Austrian wine" , "Dry white wine (German)" .)