Turned up by car - you can just hear the engine (4)
I believe the answer is:
purr
'you can just hear the engine' is the definition.
I can't judge whether this defines the answer.
'turned up by car' is the wordplay.
'turned' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'car' becomes 'RR' (Rolls Royce).
'up' written backwards gives 'pu'.
'pu'+'rr'='PURR'
Can you help me to learn more?
(Other definitions for purr that I've seen before include "Show pleasure" , "Sound of happy animal" , "Sound like happy cat" , "Tiddles' sound" , "Noise of a contented feline" .)