New priest, maybe, yet most mature (6)
I believe the answer is:
ripest
'most mature' is the definition.
('ripe' can be a synonym of 'mature')
'new priest' is the wordplay.
'new' indicates anagramming the letters.
'priest' is an anagram of 'RIPEST'.
'maybe yet' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ripest that I've seen before include "Most mature and ready to eat" , "to be furthest from green" , "Most ready for harvesting or eating" .)