Say 'no' (6)
I believe the answer is:
refuse
'say no' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for refuse that I've seen before include "Decline to accept - rubbish" , "Say no to - waste" , "Don't take the rubbish" , "Deny; rubbish" , "Decline to jump (a fence)" .)