End fuel upset - that's necessary (7)
I believe the answer is:
needful
'necessary' is the definition.
('needful' can be similar in meaning to 'necessary')
'end fuel upset' is the wordplay.
'upset' indicates anagramming the letters.
'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" , "Requisite, necessary" , "Nude elf (anag.)" , "Necessary for nude elf" , "Poverty-stricken" .)