Opinion one can always go back on? (5)

I believe the answer is:

tenet

I'm a little stuck... Click here to teach me more about this clue!

'opinion' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for tenet that I've seen before include "Key principle" , "A palindromic doctrine or creed" , "Article of belief" , "Belief held as true" , "A belief or creed - a palindrome" .)

I've seen this clue in The Globe and Mail and The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.