It's very much used as a prefix (5)

I believe the answer is:

ultra

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'it's very' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for ultra that I've seen before include "Violet's predecessor" , "lacking moderation" , "Saharan dweller" , "(As prefix) beyond or extreme" , "Prefix meaning extreme or excessive" .)

I've seen this clue in The Globe and Mail.

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