Orthodox religious community on Iona left (12)
I believe the answer is:
conventional
'orthodox' is the definition.
('conventional' can be similar in meaning to 'orthodox')
'religious community on iona left' is the wordplay.
'religious community' becomes 'convent' (I've seen this before).
'on' means one lot of letters go next to another.
'left' becomes 'l' (common abbreviation).
'convent'+'iona'+'l'='CONVENTIONAL'
(Other definitions for conventional that I've seen before include "Standard" , "Customary; not 9 ac" , "Customary, mainstream" .)