Significant, momentous (8)
I believe the answer is:
historic
This clue is a double definition.
'significant' is the first definition.
(of historic importance)
'momentous' is the second definition.
(I've seen this in another clue)
(Other definitions for historic that I've seen before include "Epoch-making" , "sort of present" , "Pertaining to what is important in the past" , "Connecting with old times" , "Landmark; famous" .)