Fifth such journalist to become an infiltrator? (9)
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?" .)