Undue speed (5)
I believe the answer is:
haste
'undue speed' is the definition.
(haste is hurrying or rushing)
This is all the clue.
(Other definitions for haste that I've seen before include "impetuosity" , "Inconsiderate speed" , "The more of it, the less speed sometimes" , "Heats (anag.)" , "''More . . . . ., less speed'' (5)" .)