Thrifty form of fuel for the car (7)
I believe the answer is:
careful
'thrifty' is the definition.
(synonyms)
'form of fuel for the car' is the wordplay.
'form of' indicates an anagram (another form of the same letters).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'fuel' with letters rearranged gives 'eful'.
'eful' put after 'car' is 'CAREFUL'.
(Other definitions for careful that I've seen before include "Thorough" , "Diligent" , "Attentive" , "'Prudent, cautious (7)'" , "Tight-fisted" .)