Clear to race having changed throttle (11)
I believe the answer is:
accelerator
'throttle' is the definition.
(synonyms)
'clear to race having changed' is the wordplay.
'having changed' is an anagram indicator.
'clear'+'to'+'race'='cleartorace'
'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" .)