Additionally, it's all the quicker to slow you down! (5)
I believe the answer is:
haste
'slow you down' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'additionally it's all the quicker' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
The remaining letters 'has' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for haste that I've seen before include "Speed, hurry" , "make speed" , "The more of it, the less speed sometimes" , "Rush; urgency" , "Hurriedness" .)