Excessively quick (5)
I believe the answer is:
hasty
'excessively quick' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hasty that I've seen before include "Too hurried, careless" , "Impetuous - quick-tempered" , "Hurried; without thinking" , "Rashly quick" , "Rushed; rash" .)