To blend it into the fuel mixture is useless (6)
I believe the answer is:
futile
'useless' is the definition.
(similar in meaning)
'to blend it into the fuel mixture' is the wordplay.
'to blend' is an anagram indicator (I've seen this in other clues).
'into' means one lot of letters goes inside another.
'mixture' indicates an anagram.
'it' anagrammed gives 'ti'.
'fuel' with letters rearranged gives 'fule'.
'ti' going inside 'fule' is 'FUTILE'.
'is' is the link.
(Other definitions for futile that I've seen before include "Useless, a waste of effort" , "If lute is altered it becomes ineffectual." , "Ineffective" , "Useless, producing no results" , "Useless, pointless" .)