Relax and open the car window (4,4)

I believe the answer is:

wind down

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'relax' is the definition.
(to wind down is to relax or chill out)

I don't understand how the remainder of the clue works.

(Other definitions for wind down that I've seen before include "Relax after stress" , "Chill" , "Relax after activity" .)

I've seen this clue in the Evening Standard.

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