A definite (yet indefinite!) refusal (5)
I believe the answer is:
never
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for never that I've seen before include "Strong negative" , "On no account" , "'It's now or - - ' (14 song)" , "At no time in the past or future" , "At no time whatever" .)