P for Porsche fanatic? (10)

I believe the answer is:

petrolhead

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'p' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see an association between them that I don't see?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for petrolhead that I've seen before include "Car lover" , "Motoring fanatic" .)

I've seen this clue in The Times.

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