It's a fag preparing grapes (6)
I believe the answer is:
gasper
'it's a fag' is the definition.
(slang term for a cigarette)
'preparing grapes' is the wordplay.
'preparing' is an anagram indicator (the same letters prepared differently).
'grapes' with letters rearranged gives 'GASPER'.
(Other definitions for gasper that I've seen before include "fag" , "Cigarette; one catching breath" , "Ciggie" , "Grapes (anag) -- cigarette (slang)" , "who can hardly get the words out?" .)