Pioneer priest on island adopting strange rites (12)
I believe the answer is:
frontiersman
'pioneer' is the definition.
(frontiersman is a kind of pioneer)
'priest on island adopting strange rites' is the wordplay.
'priest' becomes 'fr'.
'on island' becomes 'onman' (I can't justify this - if you can you should give a lot more credence to this answer).
'adopting' indicates putting letters inside.
'strange' is an anagram indicator.
'rites' with letters rearranged gives 'tiers'.
'onman' going around 'tiers' is 'ontiersman'.
'fr'+'ontiersman'='FRONTIERSMAN'
Can you help me to learn more?
(Other definitions for frontiersman that I've seen before include "his position is borderline" , "Border dweller" , "pioneer" .)