Motor takes on fresh fuel, being cautious (7)
I believe the answer is:
careful
'cautious' is the definition.
('careful' is associated in meaning with 'cautious')
'motor takes on fresh fuel' is the wordplay.
'motor' becomes 'car' (I've seen this before).
'takes on' says to put letters next to each other.
'fresh' indicates anagramming the letters.
'fuel' with letters rearranged gives 'eful'.
'car'+'eful'='CAREFUL'
'being' is the link.
(Other definitions for careful that I've seen before include "Tight-fisted" , "Painstaking" , "Guarded" , "Exercising caution" , "Diligent" .)