This speed, being high, is not raised (3)
I believe the answer is:
ton
'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" .)