Wary of motoring organisation backing fuel modification (7)
I believe the answer is:
careful
'wary' is the definition.
('careful' can be similar in meaning to 'wary')
'motoring organisation backing fuel modification' is the wordplay.
'motoring organisation' becomes 'rac'.
'backing' is a reversal indicator.
'modification' is an anagram indicator.
'rac' reversed gives 'car'.
'fuel' anagrammed gives 'eful'.
'car'+'eful'='CAREFUL'
'of' acts as a link.
(Other definitions for careful that I've seen before include "Thrifty" , "'Prudent, cautious (7)'" , "Watch it!" , "Guarded" , "Attentive" .)