Impulsive speed (5)
I believe the answer is:
haste
'impulsive speed' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for haste that I've seen before include "The more of it, the less speed, it is said" , "dispatch" , "furious activity" , "''More . . . . ., less speed'' (5)" , "Rushing (but with less speed?)" .)