The most common and most likely answer to this crossword clue is the 6 plus 7 letter word REBUFF RE-BUFF
R
E
B
U
F
F
R
E
-
B
U
F
F
Buddy explains!
The straight part
We know that "Say no" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case say no = rebuff re-buff.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.