Celts are strange, that's most apparent (8)

I believe the answer is:

clearest

'most apparent' is the definition.
('clear' can be a synonym of 'apparent')

'celts are strange' is the wordplay.
'strange' indicates an anagram.
'celts'+'are'='celtsare'
'celtsare' is an anagram of 'CLEAREST'.

'that's' is the link.

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