Astronomical bear (4)
I believe the answer is:
ursa
'astronomical bear' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ursa that I've seen before include "Major and Minor, the Bear constellations" , "Bear (constellation)" , "(In the sky) Major or Minor" , "stars" , "Constellation (Major or Minor)" .)