Wine when it turns (4)
I believe the answer is:
asti
'wine' is the definition.
(Italian wine)
'when it turns' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'turns' is a reversal indicator.
'it' reversed gives 'ti'.
'as'+'ti'='ASTI'
(Other definitions for asti that I've seen before include "- spumante, It. wine" , "Piedmont wine city, SE of Turin" , "Italian white wine" , "Italian town producing sparkling wine" , "Sparkling drink" .)