End fuel recycling as required (7)
I believe the answer is:
needful
'required' is the definition.
('needful' can be a synonym of 'required')
'end fuel recycling' is the wordplay.
'recycling' is an anagram indicator.
'end'+'fuel'='endfuel'
'endfuel' anagrammed gives 'NEEDFUL'.
'as' acts as a link.
(Other definitions for needful that I've seen before include "Requisite, necessary" , "Indispensable" , "Nude elf (anag.)" , "Required" , "Necessary for nude elf" .)