A speed at which not to reverse! (3)
I believe the answer is:
ton
'a speed' is the definition.
(I've seen this before)
'not to reverse' is the wordplay.
'to reverse' shows that the letters should be reversed in order.
'not' written backwards gives 'TON'.
'at which' acts as a link.
(Other definitions for ton that I've seen before include "1000 kilos" , "High-speed" , "People of fashion" , "20 cwt" , "Speed" .)