A requirement one’s retained for cake flavouring, perhaps (7)
I believe the answer is:
aniseed
'for cake flavouring perhaps' is the definition.
(aniseed is used as a flavouring)
'a requirement one's retained' is the wordplay.
'requirement' becomes 'need' (synonyms).
'one's' becomes 'I's' (Roman numeral).
'retained' means one lot of letters goes inside another (inserted letters are held or retained).
'need' placed around 'is' is 'niseed'.
'a'+'niseed'='ANISEED'
(Other definitions for aniseed that I've seen before include "Die sane (anag)" , "Flavouring of sambuca" , "Liquorice flavouring, used in cookery" , "Licorice-like flavour" , "Pernod flavouring" .)