Rats upset a student of the stars (6)
I believe the answer is:
astral
'of the stars' is the definition.
(relating to stars or astronomy)
'rats upset a student' is the wordplay.
'upset' indicates anagramming the letters.
'student' becomes 'l' (as in L-plates for learner drivers).
'rats' is an anagram of 'astr'.
'astr'+'a'+'l'='ASTRAL'
(Other definitions for astral that I've seen before include "To do with stars on altars" , "Like the stars" , "Pertaining to the stars" , "Characteristic of stars" , "Related to the stars" .)