No, and that's final (5)
I believe the answer is:
never
'no' is the definition.
Both the answer and definition are adverbs. Perhaps there's a link between them I don't understand?
'and that's final' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'that's final' becomes 'ever' (I can't justify this - if you can you should believe this answer much more).
'n'+'ever'='NEVER'
Can you help me to learn more?
(Other definitions for never that I've seen before include "Do not give up!" , "Emphatic negative" , "Under no circumstances" , "' - - Say - - Again': Bond film" , "Certainly not!" .)