It's quite informal so to speak (5)

I believe the answer is:

slang

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'speak' is the definition.
(I know this)

I don't understand the remainder of the clue.

(Other definitions for slang that I've seen before include "Informal language - jargon" , "Patois" , "Insult" , "Informal language used by a set of people" , "abuse" .)

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

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