It's not in a mouthpiece (3)

I believe the answer is:

out

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

'it's not in' is the definition.
('out' is the opposite to 'in')

'a mouthpiece' is the wordplay.
'OUT' is hidden within the letters.
However, I'm unsure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for out that I've seen before include "Means of escape" , "Dismissed, in cricket" , "sleeping" , "in the shops" , "Free" .)

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

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