Religious belief almost undermining star, oddly (6)
I believe the answer is:
sacred
'religious' is the definition.
(similar in meaning)
'belief almost undermining star oddly' is the wordplay.
'belief' becomes 'credo' (credo is a kind of belief).
'almost' means to remove the last letter (most of the word but not all of it).
'undermining' is a charade indicator (letters next to each other).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'star' are 'sa'.
'credo' with its last letter taken away is 'cred'.
'cred' put after 'sa' is 'SACRED'.
(Other definitions for sacred that I've seen before include "Like gravestones maybe" , "'Hallowed, holy (6)'" , "Sanctified" , "Hallowed, consecrated" , "'Consecrated, holy (6)'" .)