Be off butter? That's extreme (5)

I believe the answer is:

utter

I'm a little stuck... Click here to teach me more about this clue!

'extreme' is the definition.
(I've seen this before)

'be off butter?' is the wordplay.
'UTTER' is hidden in amongst the letters.
But, I'm uncertain how the hidden word is indicated.

'that's' acts as a link.

Can you help me to learn more?

(Other definitions for utter that I've seen before include "Say the words" , "Articulate, express" , "talk" , "Total - speak" , "Say - complete" .)

I've seen this clue in the Sydney Morning Herald.

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