Devotedly unshakeable belief to the greatest degree (10)

I believe the answer is:

faithfully

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'devotedly' is the definition.
(thesaurus)

'unshakeable belief to the greatest degree' is the wordplay.
'unshakeable belief' becomes 'faith' (I've seen this before).
'to the greatest degree' becomes 'fully' (I can't explain this - if you can you should believe this answer much more).
'faith'+'fully'='FAITHFULLY'

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I've seen this clue in The Irish Times.

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