Island that's good for religious leader (6)
I believe the answer is:
mullah
'for religious leader' is the definition.
(I've seen this before)
'island that's good' is the wordplay.
'island' becomes 'mull' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that's good' becomes 'ah' (sound made when satisfied or content).
'mull'+'ah'='MULLAH'
Can you help me to learn more?
(Other definitions for mullah that I've seen before include "Head of a mosque" , "Muslim teacher" , "Muslim learned in theology and sacred law" , "Islamic scholar" , "Muslim learned in Islamic theology and law" .)