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 don't understand how they can define each other.
'hastens with it' is the wordplay.
'hastens' becomes 'sped' (I can't justify this - if you can you should believe this answer much more).
'with' means one lot of letters goes inside another.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'sped' placed around 'e' is 'SPEED'.
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" .)