Time in entirety, possibly (8)
I believe the answer is:
eternity
'time' is the definition.
(eternity is a kind of time)
'entirety possibly' is the wordplay.
'possibly' is an anagram indicator.
'entirety' is an anagram of 'ETERNITY'.
'in' is the link.
(Other definitions for eternity that I've seen before include "Evermore" , "Endless period" , "That's a very long time" , "Wolfgang Amadeus -" , "An extremely long time" .)