Columnist, say, who sets things straight on the radio (6)
I believe the answer is:
writer
'columnist say' is the definition.
(columnist is a kind of writer)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for writer that I've seen before include "Author; correspondent" , "See 1, 12, 15, 23 across and 3, 6, 9 down" , "Quiller" , "Memorandum could one produce?" , "See 4" .)