Crossword Genius

One less heavy on petrol? (7)

Ross

I believe the answer is:

lighter

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'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.

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(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" .)

I've seen this clue in the Evening Standard.
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