Sounds like a support claim (6)
I believe the answer is:
allege
'claim' is the definition.
(to allege something is to make a claim)
'sounds like a support' is the wordplay.
I cannot quite understand how this works, but
'like' could be 'eg' (I've seen this in other clues) and 'eg' is present in the answer.
'a' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for allege that I've seen before include "Assert - maintain" , "hold" , "Declare without being able to prove" , "Aver (without proof)" , "Declare or accuse without proof" .)