End fuel blending as required (7)
I believe the answer is:
needful
'required' is the definition.
(I know that required can be written as needful)
'end fuel blending' is the wordplay.
'blending' is an anagram indicator (letters blended or mixed into a new form).
'end'+'fuel'='endfuel'
'endfuel' is an anagram of 'NEEDFUL'.
'as' is the link.
(Other definitions for needful that I've seen before include "Necessary for nude elf" , "Nude elf (anag.)" , "Poverty-stricken" , "Requisite, necessary" , "Required" .)