I'd return, else change type of fuel (6)
I believe the answer is:
diesel
'type of fuel' is the definition.
(I've seen this in another clue)
'i'd return else change' is the wordplay.
'return' shows that the letters should be reversed in order.
'change' indicates anagramming the letters.
'id' reversed gives 'di'.
'else' anagrammed gives 'esel'.
'di'+'esel'='DIESEL'
(Other definitions for diesel that I've seen before include "See lid for the engine" , "Type of engine" , "Derv" , "Kind of engine" , "Eponymous German engineer" .)