One writing lines, say, is twiddling pens (8)
I believe the answer is:
essayist
'one writing' is the definition.
(someone who writes essays)
'lines say is twiddling pens' is the wordplay.
'pens' indicates a hidden word (pen can mean to contain or cage).
'ESSAYIST' is hidden within 'lines say is twiddling'.
(Other definitions for essayist that I've seen before include "Lamb, for example" , "Bacon maybe" , "Writer of pieces on various subjects" , "Writer of short compositions" , "Montaigne, say" .)