Member engaged in bitter claim? (6)
I believe the answer is:
allege
'claim?' is the definition.
(to allege something is to make a claim)
'member engaged in bitter' is the wordplay.
'member' becomes 'leg' (member can mean a limb).
'engaged in' indicates putting letters inside.
'bitter' becomes 'ale' (type of beer or bitter).
'leg' going inside 'ale' is 'ALLEGE'.
(Other definitions for allege that I've seen before include "Accuse without established proof" , "Assert - maintain" , "hold" , "Declare without being able to prove" , "Declare or assert without proof" .)