Very fast, but possibly not on the way to town (3-2)

I believe the answer is:

ton-up

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'very' is the definition.
Both the answer and definition are adjectives.
Maybe there's an association between them I don't understand?

I cannot understand how the rest of the clue works.

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(Other definitions for ton-up that I've seen before include "Fond of travelling at high speed" , "Travelling at one hundred mph" , "Going at over 100mph" , "Going at 100 mph" , "At one-hundred mph" .)

I've seen this clue in the Evening Standard.

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