One's restricted by a lack in flavour (7)
I believe the answer is:
aniseed
'flavour' is the definition.
(I've seen this before)
'one's restricted by a lack' is the wordplay.
'one's' becomes 'is' (roman numeral).
'restricted by' is an insertion indicator (I've seen this in other clues).
'lack' becomes 'need' (I've seen this before).
'a'+'need'='aneed'
'is' going into 'aneed' is 'ANISEED'.
'in' acts as a link.
(Other definitions for aniseed that I've seen before include "Die sane (anag)" , "Liquorice flavour" , "Eased in (anag)" , "Licorice-flavoured sweets" , "Strong flavour" .)