Liqueur is filling a requirement (7)
I believe the answer is:
aniseed
'liqueur' is the definition.
(aniseed liqueur is a type of liqueur)
'is filling a requirement' is the wordplay.
'filling' is an insertion indicator.
'requirement' becomes 'need' ('need' can be a synonym of 'requirement').
'a'+'need'='aneed'
'is' put inside 'aneed' is 'ANISEED'.
(Other definitions for aniseed that I've seen before include "Flavouring of sambuca" , "Strong herbal flavouring" , "Strong flavour" , "Liquorice-flavoured seeds" , "Pernod flavouring" .)