Not over high speed (3)
I believe the answer is:
Here is my best explanation:
'high speed' is the definition.
'ton' can be an answer for 'speed' (I've seen this before). I am not sure about the 'high' bit.
'not over' is the subsidiary indication.
'over' says the letters should be written in reverse.
'not' backwards is 'ton'.
(Other definitions for ton that I've seen before include "2240lbs", "Not a unit of weight", "Fashionable", "Weight - fashion (French)", "U nit of weight", "See 25", "Heavyweight", "Hundred", "Two thousand, two hundred and forty lb" and "Unit of weight equivalent to 20 cwts".)