Say no to litter (6)
I believe the answer is:
refuse
This clue is a double definition.
'say no' is the first definition.
(thesaurus)
'litter' is the second definition.
(I've seen this before)
'to' is the link.
(Other definitions for refuse that I've seen before include "Turn down; waste" , "Decline to accept - waste" , "Trash" , "Decline waste material" , "Junk" .)