Expressing refusal (8)
I believe the answer is:
negative
'expressing refusal' is the definition.
('no' is answering in the negative)
This is all the clue.
(Other definitions for negative that I've seen before include "This required for development" , "Opposite of affirmative" , "Expressing refusal, denial or disagreement" , "Less than zero, minus" , "The photographic film expresses refusal" .)