Assert without proof member into booze (6)
I believe the answer is:
allege
'assert without proof' is the definition.
(I've seen this before)
'member into booze' is the wordplay.
'member' becomes 'leg' (leg is a kind of member).
'into' means one lot of letters goes inside another.
'booze' becomes 'ale' (type of alcohol).
'leg' going within 'ale' is 'ALLEGE'.
(Other definitions for allege that I've seen before include "Declare or accuse without proof" , "Aver (without proof)" , "Declare or assert without proof" , "Aver - maintain" , "Declare without being able to prove" .)