Very fast? This suggests not (3-2)

I believe the answer is:

ton-up

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'not' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for ton-up that I've seen before include "Travelling at one hundred mph" , "fast!" , "Travelling at 100 mph" , "going quickly" , "Going at 100 mph" .)

I've seen this clue in The Times.

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