Just a bad molar (5)

I believe the answer is:

moral

'just' is the definition.
('moral' can be a synonym of 'just')

'a bad molar' is the wordplay.
'a bad' indicates an anagram.
'molar' anagrammed gives 'MORAL'.

(Other definitions for moral that I've seen before include "Righteous" , "ethical standard" , "Pertaining to right and wrong conduct" , "ethical principle" , "Lesson of a fable" .)

I've seen this clue in the Evening Standard.

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