Opinion one can always go back on? (5)

I believe the answer is:

tenet

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'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 Guardian and The Globe and Mail.

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