Excessive speed (5)
I believe the answer is:
haste
'excessive speed' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for haste that I've seen before include "Urgent hurry" , "''Marry in . . . . ., repent at leisure''" , "Rush; urgency" , "Undue speed" , "furious activity" .)