Astronomer Hubble
I believe the answer is:
edwin
'astronomer hubble' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for edwin that I've seen before include "-- Hubble (astronomer)" , "Sir - Landseer, artist" , "and 21 Across: Dickens character" , "Old king's" , "old king" .)