Old spiritual leader good with God mostly (6)
I believe the answer is:
gandhi
'old spiritual leader' is the definition.
'Gandhi' can be an answer for 'leader' (Gandhi is an example). I'm not sure about the 'old spiritual' bit.
'good with god mostly' is the wordplay.
'good' becomes 'g' (abbreviation).
'with' becomes 'and'.
'god mostly' becomes 'hi' (I can't justify this - if you can you should give a lot more credence to this answer).
'g'+'and'+'hi'='GANDHI'
Can you help me to learn more?
(Other definitions for gandhi that I've seen before include "Leader of Indian independence movement" , "Famous Indian politician" , "Peaceful protester" , "Indian political dynasty" , "hunger-striker" .)