Speed zero in a car, since it’s in mothballs (7)

I believe the answer is:

camphor

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'since it's in mothballs' is the definition.
I can't judge whether this definition defines the answer.

'speed zero in a car' is the wordplay.
'speed' becomes 'mph' (miles per hour).
'zero' becomes 'o'.
'in' indicates putting letters inside.
'mph'+'o'='mpho'
'mpho' placed into 'car' is 'CAMPHOR'.

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(Other definitions for camphor that I've seen before include "Liniment" , "Mothball ingredient" , "Moth-repellent substance" , "Aromatic substance used as an insect repellent" , "Constituent of mothballs" .)

I've seen this clue in the Evening Standard.

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