The most common and most likely answer to this crossword clue is the 6 letter word REFUSE
I've seen this crossword clue in The Times
R
E
F
U
S
E
Buddy explains!
The straight part
We know that "Say no to/junk" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case say no to/junk = refuse.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does REFUSE mean?
REFUSE noun- Collectively, items or material that have been discarded; rubbish, garbage.
- Discarded, rejected.
- Refusal
- To decline (a request or demand).
- To decline a request or demand, forbear; to withhold permission.
- To throw back, or cause to keep back (as the centre, a wing, or a flank), out of the regular alignment when troops are about to engage the enemy.
- To disown.
- To melt again.