Relating to the universe; vast (6)
I believe the answer is:
cosmic
I believe this is a double definition.
'relating to the universe' is the first definition.
(I've seen this before)
'vast' is the second definition.
(I've seen this in another clue)
(Other definitions for cosmic that I've seen before include "Great" , "To do with the whole universe" , "Of the entire universe" , "Huge" , "Originating in outer space" .)