Clerics a Yankee has converted on coral island (10)
I believe the answer is:
ayatollahs
'clerics' is the definition.
(Iranian religious leaders)
'a yankee has converted on coral island' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'converted' indicates an anagram.
'on' indicates putting letters inside (as in clothing 'on' a person).
'coral island' becomes 'atoll' (I've seen this before).
'has' anagrammed gives 'ahs'.
'ahs' placed around 'atoll' is 'atollahs'.
'a'+'y'+'atollahs'='AYATOLLAHS'
(Another definition for ayatollahs that I've seen is " Religious leaders".)