Astronomer Carl ____ (5)
I believe the answer is:
sagan
'astronomer carl' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sagan that I've seen before include "French authoress" , "Author of Bonjour Tristesse, d. 2004" , "US scientist" , "Astronomer" , "French author" .)