It's good for the car, but bad for the cylinders (6)
I believe the answer is:
carbon
'bad for the cylinders' is the definition.
I can't judge whether this definition defines the answer.
'it's good for the car' is the wordplay.
'it's good' becomes 'bon' (I've seen this in other clues).
'for' says to put letters next to each other (I've seen this in other clues).
'bon' put after 'car' is 'CARBON'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for carbon that I've seen before include "Charcoal is a form of this element" , "Abundant chemical element, symbol C" , "Eg, diamond or graphite" , "Element in pollutant" , "Elementary" .)