To mature, a bit of a writer needs a pen (5)
I believe the answer is:
ripen
'to mature' is the definition.
(ripening is a kind of maturing)
'a bit of a writer needs a pen' is the wordplay.
'a bit of a writer needs' becomes 'ri' (I can't justify this - if you can you should believe this answer much more).
'ri'+'pen'='RIPEN'
Can you help me to learn more?
(Other definitions for ripen that I've seen before include "Become ready for harvesting" , "Become mature" , "Become ready to pick" , "Grow to maturity like fruit" , "Mature, develop" .)