Avoid car accident when temperature drops (4)

I believe the answer is:

shun

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

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

(Other definitions for shun that I've seen before include "Deliberately avoid" , "Ostracise" , "Avoid, keep away from" , "military command" , "Send to Coventry" .)

I've seen this clue in The Guardian.

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