Crossword Genius

This speed, being high, is not raised (3)

Ross

I believe the answer is:

ton

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'this speed being high' is the definition.
'ton' can be an answer for 'speed' (I've seen this before). I am unsure of the rest of the definition.

'not raised' is the wordplay.
'raised' says the letters should be written backwards (going up in a down clue).
'not' in reverse letter order is 'TON'.

'is' acts as a link.

Can you help me to learn more?

(Other definitions for ton that I've seen before include "U nit of weight" , "Twenty cwt" , "A slang 100 mph" , "20 cwt" , "Unit of weight equivalent to 2240 pounds" .)

I've seen this clue in the Evening Standard.
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