Claim litre, say, in drink (6)
I believe the answer is:
allege
'claim' is the definition.
(thesaurus)
'litre say in drink' is the wordplay.
'litre' becomes 'l' ('l' can be a synonym of 'litre').
'say' becomes 'eg'.
'in' indicates putting letters inside.
'drink' becomes 'ale' (ale is a kind of drink**).
'l'+'eg'='leg'
'leg' going into 'ale' is 'ALLEGE'.
(Other definitions for allege that I've seen before include "Assert - maintain" , "Swear" , "Declare without being able to prove" , "Cite in discussion" , "Declare or accuse without proof" .)