Clear to race having changed throttle (11)
I believe the answer is:
'throttle' is the definition.
'clear to race having changed' is the wordplay.
'having changed' is an anagram indicator.
'cleartorace' anagrammed gives 'ACCELERATOR'.
(Other definitions for accelerator that I've seen before include "Driver's foot pedal" , "Pedal controlling vehicle's speed" , "It increases road speed" , "Substance speeding up a chemical process" , "Throttle" .)