Mature writer on the teaching of religion (5)
I believe the answer is:
ripen
'mature' is the definition.
(to ripen is to mature)
'writer on the teaching of religion' is the wordplay.
'writer' becomes 'pen' (something which writes).
'on' says to put letters next to each other.
'the teaching of religion' becomes 'RI' (Religious Instruction).
'pen' put after 'ri' is 'RIPEN'.
(Other definitions for ripen that I've seen before include "Get ready to open" , "Become ready for eating" , "Mature, develop" , "Grow mature like fruit" , "Grow older" .)