Mature writer follows religious instruction (5)
I believe the answer is:
ripen
'mature' is the definition.
(ripening is a kind of maturing)
'writer follows religious instruction' is the wordplay.
'writer' becomes 'pen' (I've seen this before).
'follows' says to put letters next to each other.
'religious instruction' becomes 'ri' (abbreviation).
'pen' after 'ri' is 'RIPEN'.
(Other definitions for ripen that I've seen before include "Get ready for 2" , "Grow to maturity like fruit" , "Come to maturity like fruit" , "Mature, age" , "Become ready to pick" .)