Red wine, gallon drunk by debauchee (5)
I believe the answer is:
rouge
'red wine' is the definition.
(French term for red wine)
'gallon drunk by debauchee' is the wordplay.
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'drunk by' is an insertion indicator.
'debauchee' becomes 'roue' (roue is a kind of debauchee).
'g' put inside 'roue' is 'ROUGE'.
(Other definitions for rouge that I've seen before include "Cosmetic powder" , "Powder for the cheeks" , "Ferric oxide powder" , "Make-up item" , "Polish used by jewellers" .)