Religious figure's friend had converted (6)
I believe the answer is:
buddha
'religious figure's' is the definition.
(founder of Buddhism)
'friend had converted' is the wordplay.
'friend' becomes 'bud' (short for buddy).
'converted' indicates anagramming the letters.
'had' anagrammed gives 'dha'.
'bud'+'dha'='BUDDHA'
(Other definitions for buddha that I've seen before include "Founder of an Indian religion and philosophy" , "God" , "Siddhartha Gautama (c 563-483 BC)" , "Founder of a world religion" , "religious effigy" .)