Mixing it into the wrong fuel is useless! (6)
I believe the answer is:
futile
'useless' is the definition.
(similar in meaning)
'mixing it into the wrong fuel' is the wordplay.
'mixing' indicates an anagram.
'into' indicates putting letters inside.
'the wrong' indicates anagramming the letters.
'it' with letters rearranged gives 'ti'.
'fuel' is an anagram of 'fule'.
'ti' inserted inside 'fule' is 'FUTILE'.
'is' acts as a link.
(Other definitions for futile that I've seen before include "Useless, pointless" , "Fruitless" , "In vain, a waste of time" , "Useless or vain" , "Useless, a waste of effort" .)