Writer - the fifth may be a traitor (9)
I believe the answer is:
columnist
'writer' is the definition.
(I know that columnist is a type of writer)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for columnist that I've seen before include "Waters or O'Toole, say" , "writer" , "Contributor to newspaper" , "Journalist making regular contributions" , "leader?" .)