Knock back (6)
(Other definitions for rebuff that I've seen before include "Snub or contemptuous refusal" , "Deliberate discourteous act" , "Reject abruptly" , "Refuse" , "Abrupt or ungracious rejection" .)
'knock back' is the definition.
(I know that knock back can be written as refuse)
This is all the clue.
(Other definitions for refuse that I've seen before include "deadwood" , "Turn away" , "Junk" , "Say no to - waste" , "Decline to accept garbage" .)