Engine booster (5)
I believe the answer is:
turbo
'engine booster' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for turbo that I've seen before include "source of power" , "Charger improves a car's power" , "Engine supercharger" , "found in car" , "Type of supercharger using engines exhaust gases" .)