Say no to waste (6)
I believe the answer is:
refuse
I believe this is a double definition.
'say no to' is the first definition.
(to say no to something is to refuse it)
'waste' is the second definition.
(I know that waste can be written as refuse)
(Other definitions for refuse that I've seen before include "Decline to take the rubbish" , "Decline to jump; rubbish" , "Waste, garbage" , "Say no to - waste" , "Don't take the rubbish" .)