Major in astronomy? (4)
I believe the answer is:
ursa
'major in astronomy?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ursa that I've seen before include "(In the sky) Major or Minor" , "(& 19 across) Plough constellation" , "- Major, - Minor, prominent constellations" , "'Major and Minor, constellations (4)'" , "stars" .)