A speed at which not to reverse! (3)
I believe the answer is:
ton
'a speed at which' is the definition.
'ton' can be an answer for 'a speed' (I've seen this before). I am unsure of the remainder of the definition.
'not to reverse' is the wordplay.
'to reverse' shows that the letters should be reversed in order.
'not' back-to-front is 'TON'.
Can you help me to learn more?
(Other definitions for ton that I've seen before include "1000 kilos" , "High-speed" , "People of fashion" , "20 cwt" , "Speed" .)