Crossword Genius

In the car, it serves a shady purpose (5)

Ross

I believe the answer is:

visor

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

I can't explain the remainder of the clue.

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(Other definitions for visor that I've seen before include "Screen to filter glare" , "Part of an armour helmet" , "Movable shield on helmet" , "Moveable piece of helmet" , "Peak of a cap" .)

I've seen this clue in the Evening Standard.
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