Make idle one react differently, slowing down (12)
I believe the answer is:
deceleration
'slowing down' is the definition.
(the opposite of acceleration)
'make idle one react' is the wordplay.
'make' indicates anagramming the letters.
'idle'+'one'+'react'='idleonereact'
'idleonereact' anagrammed gives 'DECELERATION'.
'differently' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
(Other definitions for deceleration that I've seen before include "Slowing of velocity" , "Process of slowing down" .)