Betray Cosa Nostra, having ripped off leader's liqueur (7)
I believe the answer is:
'liqueur' is the definition.
(ratafia is a kind of liqueur)
'betray cosa nostra having ripped off leader's' is the wordplay.
'betray' becomes 'rat' (to rat someone out is to betray them).
'cosa nostra' becomes 'mafia' (Cosa Nostra is an Italian term for the mafia).
'having ripped off leader' means to remove the first letter (the letter which leads the word is removed).
'mafia' with its first letter taken off is 'afia'.
(Other definitions for ratafia that I've seen before include "A liqueur; almond flavouring" , "Almond biscuit/liqueur" , "Almond-flavoured biscuit - at a fair (anag)" , "Almond liqueur, biscuit" , "Almond flavoured biscuit at a fair (anag)" .)