Crossword Genius

In a car, one giving voice to contempt? (6)

Ross

I believe the answer is:

hooter

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'in' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I can't explain the remainder of the clue.

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(Other definitions for hooter that I've seen before include "it smells" , "Factory siren" , "Uphill stone pusher (Gk. myth)" , "Gnomon" , "It makes a loud noise" .)

I've seen this clue in the Evening Standard.
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