Crossword Genius

Fifth such journalist to become an infiltrator? (9)

Ross

I believe the answer is:

columnist

'fifth such journalist' is the definition.
'columnist' can be an answer for 'journalist' (columnist is a kind of journalist). I'm not sure about the 'fifth such' bit.

I cannot understand how the remainder of the clue works.

(Other definitions for columnist that I've seen before include "Contributor to newspaper" , "Journalist making regular contributions" , "Waters or O'Toole, say" , "writer" , "leader?" .)

I've seen this clue in The Times.
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