Rulers upsetting stars (5)
I believe the answer is:
tsars
'rulers' is the definition.
(tsar is a kind of ruler)
'upsetting stars' is the wordplay.
'upsetting' is an anagram indicator.
'stars' with letters rearranged gives 'TSARS'.
(Other definitions for tsars that I've seen before include "Old Russian rulers" , "Russian emperors" , "Old monarchs" , "Old Russia's rulers" .)