Fundamental belief (5)
I believe the answer is:
tenet
'fundamental belief' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tenet that I've seen before include "Doctrine; principle held" , "A belief or credo - it's a palindrome" , "you may believe in it" , "Article of faith; doctrine" , "Key principle" .)