Not an irreversible belief (5)
I believe the answer is:
tenet
'belief' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tenet that I've seen before include "Doctrine, dogma, belief" , "A palindromic doctrine or creed" , "Article of belief" , "Doctrine held to be true" , "Belief held as true" .)