Necessitate being in late for assembly (6)
I believe the answer is:
entail
'necessitate' is the definition.
(entailing is a kind of necessitating)
'in late for assembly' is the wordplay.
'for assembly' indicates anagramming the letters (I've seen this in other clues).
'in'+'late'='inlate'
'inlate' anagrammed gives 'ENTAIL'.
'being' is the link.
(Other definitions for entail that I've seen before include "Involve (something) as an inevitable consequence" , "Inalienable estate" , "Call for" , "settle estates for the long term" , "If willing, this settles" .)