Rate increase (12)
I believe the answer is:
acceleration
I believe this clue is a double definition.
'rate' is the first definition.
(acceleration is a kind of rate)
'increase' is the second definition.
(I have seen 'Increase of speed' mean 'acceleration' so perhaps 'increase' could also mean 'acceleration')
(Other definitions for acceleration that I've seen before include "Rate of increase of speed" , "Speeding-up" , "The increasing of speed" , "Speeding up" .)