A clot? Speed is of the essence! (4)
I believe the answer is:
asap
'speed is of the essence' is the definition.
(ASAP means as soon as possible)
'a clot?' is the wordplay.
'clot?' becomes 'sap' (both can mean an idiot).
'a'+'sap'='ASAP'
(Other definitions for asap that I've seen before include "Notation or acronym meaning without avoidable delay" , "At the earliest opportunity (abbrev.)" , "Notation-acronym meaning without delay" , "'As quickly as you can, initially (4)'" , "At one's earliest convenience" .)