Italian white wine, a sticky, reduced by 50% (4)
I believe the answer is:
asti
'italian white wine' is the definition.
(I've seen this before)
'a sticky reduced by 50%' is the wordplay.
'reduced by 50%' suggests halving the letters.
'sticky' with half the letters taken is 'sti'.
'a'+'sti'='ASTI'
(Other definitions for asti that I've seen before include "White wine named after a town in Piedmont" , "Italian 'sparkler'" , "Italian town, famous for its wine" , "Fizzy wine" , "Italian drunk" .)