One less heavy on petrol? (7)
I believe the answer is:
lighter
'one less heavy' is the definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'less heavy')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for lighter that I've seen before include "more blonde" , "Canal boat with a flat bottom" , "Smoker's implement" , "smoker may resort to it" , "Boat for heavy loads and fire igniter" .)