Holy opening to Heaven admitted (8)
I believe the answer is:
hallowed
'holy' is the definition.
('hallowed' can be a synonym of 'holy')
'opening to heaven admitted' is the wordplay.
'opening' suggests taking the first letters.
'admitted' becomes 'allowed' (allowing is a kind of admitting).
The initial letter of 'heaven' is 'h'.
'h'+'allowed'='HALLOWED'
(Other definitions for hallowed that I've seen before include "Made holy and honoured" , "Revered" , "Like saints" , "sacrosanct" , "Honoured as holy, having lewd halo" .)