Vital contribution from knock-kneed full-back (7)
I believe the answer is:
needful
'vital' is the definition.
(thesaurus)
'contribution from knock-kneed full-back' is the wordplay.
'NEEDFUL' is hidden within the letters.
But, I am uncertain how the hidden word is indicated.
(Other definitions for needful that I've seen before include "poor" , "Nude elf (anag.)" , "Requisite, necessary" , "Necessary for nude elf" , "Poverty-stricken" .)