Evenly train writer to mature (5)
I believe the answer is:
ripen
'mature' is the definition.
(to ripen is to mature)
'evenly train writer' is the wordplay.
'evenly' indicates alternate letters (the letters in even-numbered positions).
'writer' becomes 'pen' (something which writes).
The alternating letters of 'train' are 'ri'.
'ri'+'pen'='RIPEN'
'to' acts as a link.
(Other definitions for ripen that I've seen before include "Become mature" , "Get ready to eat" , "Grow older" , "Become ready to harvest" , "Become ready for picking" .)