Complete first ultramarathon, taking to eating regularly (5)

I believe the answer is:

utter

'complete' is the definition.
(both can mean total)

'first ultramarathon taking to eating regularly' is the wordplay.
'first' suggests taking the first letters.
The initial letters of 'ultramarathon taking to eating regularly' is 'UTTER'.

(Other definitions for utter that I've seen before include "Absolutely" , "Complete madman" , "Absolute - state" , "Speak, articulate" , "Express in words" .)

I've seen this clue in The Irish Times.

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