An accelerator pressed with the heel (4)

I believe the answer is:

spur

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'the heel' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for spur that I've seen before include "offer encouragement" , "Goad - branch line" , "outcrop" , "Impel" , "Goad - incentive" .)

I've seen this clue in the Evening Standard.

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