God, in the beginning, opened doors into Nirvana (4)
I believe the answer is:
odin
'god' is the definition.
(Norse god)
'beginning opened doors into nirvana' is the wordplay.
'beginning' says to take the initial letters.
The initial letters of 'opened doors into nirvana' is 'ODIN'.
'in the' is the link.
(Other definitions for odin that I've seen before include "Chief god of Scandinavian myth" , "Supreme Scandinavian god - what a racket" , "Chief Norse god" , "Supreme Norse deity" , "Supreme god of war in Norse mythology" .)