Cautious use of fuel for the car (7)
I believe the answer is:
careful
'cautious' is the definition.
(associated in meaning)
'use of fuel for the car' is the wordplay.
'use of' indicates anagramming the letters.
'for' means one lot of letters go next to another (I've seen this in other clues).
'fuel' anagrammed gives 'eful'.
'eful' after 'car' is 'CAREFUL'.
(Other definitions for careful that I've seen before include "'Prudent, cautious (7)'" , "Tight-fisted" , "Attentive" , "Thrifty" , "Diligent" .)