Particular fuel mixture for an automobile (7)
I believe the answer is:
careful
'particular' is the definition.
(thesaurus)
'fuel mixture for an automobile' is the wordplay.
'mixture' indicates anagramming the letters.
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'an automobile' becomes 'car' (synonyms).
'fuel' with letters rearranged gives 'eful'.
'eful' put after 'car' is 'CAREFUL'.
(Other definitions for careful that I've seen before include "'Prudent, cautious (7)'" , "Attentive" , "Painstaking" , "Watch it!" , "Diligent" .)