One hastens with it (5)
I believe the answer is:
speed
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'hastens with it' is the wordplay.
I cannot really understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
The remaining letters 'sped' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for speed that I've seen before include "Rate of travel" , "Alacrity" , "Rate of progress" , "Film sensitivity to light" , "Tempo - drug" .)