Cautious of vehicle with hybrid fuel (7)
I believe the answer is:
careful
'cautious' is the definition.
('careful' is associated in meaning with 'cautious')
'vehicle with hybrid fuel' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'with' means one lot of letters go next to another.
'hybrid' indicates anagramming the letters (I've seen this in other clues).
'fuel' is an anagram of 'eful'.
'car'+'eful'='CAREFUL'
'of' acts as a link.
(Other definitions for careful that I've seen before include "Painstaking" , "'Prudent, cautious (7)'" , "Thorough" , "Diligent" , "Watch it!" .)