Guarded vehicle with spilt fuel (7)
I believe the answer is:
careful
'guarded' is the definition.
(similar in meaning)
'vehicle with spilt fuel' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'with' is a charade indicator (letters next to each other).
'spilt' is an anagram indicator.
'fuel' is an anagram of 'eful'.
'car'+'eful'='CAREFUL'
(Other definitions for careful that I've seen before include "Exercising caution" , "'Prudent, cautious (7)'" , "Diligent" , "Guarded" , "Thorough" .)