Deny King Edward, say, is making a comeback (5)
I believe the answer is:
rebut
'deny' is the definition.
(to rebut is to deny or refute)
'king edward say is making a comeback' is the wordplay.
'king edward say' becomes 'tuber' (King Edward is a type of potato).
'is making a comeback' shows that the letters should be reversed in order.
'tuber' in reverse letter order is 'REBUT'.
(Other definitions for rebut that I've seen before include "Give the lie to" , "Contradict or disprove" , "Prove to be false or incorrect" , "Deny claim" , "Scotch" .)