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' is an anagram indicator (the letters need to be in another 'form').
'for' says to put letters next to each other (I've seen this in other clues).
'fuel' is an anagram of 'eful'.
'eful' after 'car' is 'CAREFUL'.
(Other definitions for careful that I've seen before include "Thorough" , "Diligent" , "Attentive" , "'Prudent, cautious (7)'" , "Tight-fisted" .)