End fuel blending, that's necessary (7)
I believe the answer is:
needful
'necessary' is the definition.
(similar in meaning)
'end fuel blending' is the wordplay.
'blending' indicates an anagram (letters blended or mixed into a new form).
'end'+'fuel'='endfuel'
'endfuel' is an anagram of 'NEEDFUL'.
'that's' acts as a link.
(Other definitions for needful that I've seen before include "poor" , "Poverty-stricken" , "Required" , "Necessary for nude elf" , "Indispensable" .)