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