Mature writer on middle of trip (5)
I believe the answer is:
ripen
'mature' is the definition.
(ripening is a kind of maturing)
'writer on middle of trip' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'middle of' says to take the centre.
The middle letters of 'trip' are 'ri'.
'pen' put after 'ri' is 'RIPEN'.
(Other definitions for ripen that I've seen before include "Become ready to harvest" , "Become ready for picking" , "Grow mature like fruit" , "Become mature" , "Make ready" .)