American fuel - terribly handy (6)
I believe the answer is:
useful
'handy' is the definition.
(I've seen this before)
'american fuel terribly' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'terribly' is an anagram indicator.
'fuel' with letters rearranged gives 'eful'.
'us'+'eful'='USEFUL'
(Other definitions for useful that I've seen before include "Suitable for practical purpose" , "Functional and worthwhile" , "Advantageous; very able" , "Proficient; handy" , "Having a function" .)