Mature writing with Apple device? (5)
I believe the answer is:
ripen
'mature' is the definition.
(ripening is a kind of maturing)
'writing with apple device?' is the wordplay.
'writing' becomes 'pen'.
'with' says to put letters next to each other.
'apple device?' becomes 'ri' (I can't explain this - if you can you should give a lot more credence to this answer).
'pen' after 'ri' is 'RIPEN'.
(Other definitions for ripen that I've seen before include "Get ready for 2" , "Grow mature like fruit" , "Become ready to pick" , "Get ready to open" , "Come to fruition" .)