Maintain alcoholic drink is about litre, say (6)
I believe the answer is:
allege
'maintain' is the definition.
(I know that allege is a more specific form of maintain)
'alcoholic drink is about litre say' is the wordplay.
'alcoholic drink' becomes 'ale' (type of beer).
'is about' is an insertion indicator.
'litre' becomes 'L' (abbreviation).
'say' becomes 'eg' (both can mean for example).
'l'+'eg'='leg'
'ale' enclosing 'leg' is 'ALLEGE'.
(Other definitions for allege that I've seen before include "Declare without being able to prove" , "Declare or accuse without proof" , "Declare or assert without proof" , "Make a claim without proof" , "Assert, state without proof" .)