Not up to the high speed (3)
I believe the answer is:
ton
'speed' is the definition.
(I've seen this before)
'not up to the high' is the wordplay.
'not' becomes 'no' (I've seen this in other clues).
'up' is a reversal indicator (in down clue: letters go upwards).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'high' says to take the initial letters (I've seen 'highest of' mean this (top letter - down clues only)).
The initial letter of 'the' is 't'.
'no' back-to-front is 'on'.
'on' put after 't' is 'TON'.
(Other definitions for ton that I've seen before include "Fashionable" , "A slang 100 mph" , "U nit of weight" , "1000 kilos" , "20 cwt" .)