Crossword Genius

Such boys are not slow (3-2)

Ross

I believe the answer is:

ton-up

I'm a little stuck... Click here to teach me more about this clue!

'such boys are' is the definition.
The definition suggests a singular noun which matches the answer.

'not slow' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'not' is 'ton' which is within the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

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

I've seen this clue in the Evening Standard.
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