Nae sayer (4)
I believe the answer is:
scot
'nae sayer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scot that I've seen before include "Glaswegian, say" , "Stevenson was one" , "Someone from north of the border?" , "Eg, Edinburgh native" , "Native of northern Great Britain" .)