Old Celtic noble, before the hour of one, aches badly (8)

I believe the answer is:

toiseach

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'old celtic noble before the hour' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'of one aches badly' is the wordplay.
'of' becomes 'to' (I can't justify this - if you can you should believe this answer much more).
'one' becomes 'i' (Roman numeral).
'badly' indicates anagramming the letters.
'aches' anagrammed gives 'seach'.
'to'+'i'+'seach'='TOISEACH'

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(Another definition for toiseach that I've seen is " Old Celtic noble; is ace hot (anag.)".)

I've seen this clue in The Independent.

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