Not over high speed (3)

I believe the answer is:

ton

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

'high speed' is the definition.
'ton' can be an answer for 'speed' (I've seen this before). I am not certain of the 'high' bit.

'not over' is the wordplay.
'over' is a reversal indicator.
'not' reversed gives 'TON'.

Can you help me to learn more?

(Other definitions for ton that I've seen before include "See 25" , "1000 kilos" , "People of fashion" , "20cwt" , "2240lbs" .)

I've seen this clue in the Evening Standard.

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