Speculative parking on the streets (9)

I believe the answer is:

proofless

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'speculative' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see a link between them that I don't see?

'parking on the streets' is the wordplay.
I cannot quite see how this works, but
'parking' could be 'p' (abbreviation on road signs) and 'p' is located in the answer.
The remaining letters 'roofless' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Times.

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