Speed of car at event (4)
I believe the answer is:
rate
'speed' is the definition.
(rate is a kind of speed)
'of car at event' is the wordplay.
'of' says the answer is hidden in the clue.
'RATE' can be found hidden inside 'car at event'.
(Other definitions for rate that I've seen before include "Tear up in tempo" , "Be worthy of; speed" , "Estimate value of - speed" , "Assign a value to, appraise" , "standard levy" .)