Good, I am allowed a cocktail (6)
I believe the answer is:
gimlet
'a cocktail' is the definition.
(gimlet is a kind of cocktail)
'good i am allowed' is the wordplay.
'good' becomes 'g' (abbreviation).
'i am' becomes 'im'.
'allowed' becomes 'let' (I've seen this before).
'g'+'im'+'let'='GIMLET'
(Other definitions for gimlet that I've seen before include "Small tool for boring holes" , "Hole borer" , "Hole-boring tool" , "Hand tool for boring holes" , "Small hand tool - cocktail of vodka or gin with lime juice" .)