The completeness of eternity, possibly (8)
I believe the answer is:
entirety
'the completeness' is the definition.
(entirety is a kind of completeness)
'eternity possibly' is the wordplay.
'possibly' is an anagram indicator.
'eternity' with letters rearranged gives 'ENTIRETY'.
'of' acts as a link.
(Other definitions for entirety that I've seen before include "Completeness" , "The lot" , "Totality" , "Universe" , "Sum" .)