Relish swallowing a gallon of liquor (6)
I believe the answer is:
lovage
'liquor' is the definition.
(I've seen this before)
'relish swallowing a gallon' is the wordplay.
'relish' becomes 'love'.
'swallowing' is an insertion indicator.
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'a'+'g'='ag'
'love' placed around 'ag' is 'LOVAGE'.
'of' is the link.
(Other definitions for lovage that I've seen before include "Salad plant from southern Europe" , "Herb and salad plant" , "liquor" , "Parsley relative - a glove (anag)" , "F" .)