The origin of Brahminism is related to his beginnings (5)
I believe the answer is:
birth
'the origin' is the definition.
(I've seen this in another clue)
'brahminism is related to his beginnings' is the wordplay.
'beginnings' suggests taking the first letters.
The first letters of 'brahminism is related to his' is 'BIRTH'.
'of' is the link.
(Other definitions for birth that I've seen before include "Nacence" , "Parentage" , "Nativity" , "Nascence" , "Start (of something)" .)