I'd seen a modified flavouring (7)
I believe the answer is:
aniseed
'flavouring' is the definition.
(I've seen this before)
'i'd seen a modified' is the wordplay.
'modified' indicates an anagram.
'id'+'seen'+'a'='idseena'
'idseena' is an anagram of 'ANISEED'.
(Other definitions for aniseed that I've seen before include "Eased in (anag)" , "Licorice-like flavour" , "Liquorice flavouring, used in cookery" , "Plant" , "Natural flavouring" .)