Necessary; requisite (7)
I believe the answer is:
needful
I believe this is a double definition.
'necessary' is the first definition.
(similar in meaning)
'requisite' is the second definition.
(I know that requisite can be written as needful)
(Other definitions for needful that I've seen before include "Nude elf (anag.)" , "poor" , "Required" , "Poverty-stricken" , "Indispensable" .)