Star upsets a student of the stars (6)
I believe the answer is:
astral
'of the stars' is the definition.
(relating to stars or astronomy)
'star upsets a student' is the wordplay.
'upsets' indicates an anagram.
'student' becomes 'l' (as in L-plates for learner drivers).
'star' with letters rearranged gives 'astr'.
'astr'+'a'+'l'='ASTRAL'
(Other definitions for astral that I've seen before include "Characteristic of stars" , "19 [STELLAR]" , "like a star" , "Like the stars" , "Universal" .)