Such boys are not slow (3-2)
I believe the answer is:
ton-up
'such boys are' is the definition.
The definition suggests a singular noun which matches the answer.
'not slow' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'not' is 'ton' which is within the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ton-up that I've seen before include "fast!" , "Fond of travelling at high speed" , "going quickly" , "Going at over 100mph" , "Travelling at 100 mph" .)