The most common and most likely answer to this crossword clue is the 7 letter word DECLINE
I've seen this crossword clue in Independent
D
E
C
L
I
N
E
More clues leading to the result DECLINE
- Refuse to come down
- Refuse to go downhill
- Drop career after a month
- Product on sale just before Christmas maybe to become less successful?
- Refuse to go to seed
- Don’t accept deterioration
- Refuse to go through all the cases
- Drop in business after Christmas time
- Cold approach after Germany and Spain deteriorate
- Don’t accept reduction
Buddy explains!

The straight part
We know that "Say no to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case say no to = decline.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 DECLINE mean?
DECLINE noun- Downward movement, fall.
- A sloping downward, e.g. of a hill or road.
- A weakening.
- A reduction or diminution of activity.
- The act of declining or refusing something.
- To move downwards, to fall, to drop.
- To become weaker or worse.
- To bend downward; to bring down; to depress; to cause to bend, or fall.
- To cause to decrease or diminish.
- To turn or bend aside; to deviate; to stray; to withdraw.
- To refuse, forbear.
- (grammar, usually of substantives, adjectives and pronouns) To inflect for case, number and sometimes gender.
- (by extension) To run through from first to last; to repeat like a schoolboy declining a noun.
- To reject a penalty against the opposing team, usually because the result of accepting it would benefit the non-penalized team less than the preceding play.