Roman leader might cause a scare (6)
I believe the answer is:
caesar
'roman leader' is the definition.
(Julius Caesar)
'cause a scare' is the wordplay.
'cause' is an anagram indicator.
'a'+'scare'='ascare'
'ascare' is an anagram of 'CAESAR'.
'might' is the link.
(Other definitions for caesar that I've seen before include "leader who acts like a dictator" , "See 12" , "Roman dictator" , "Roman ruler" , "He was killed during the Ides of March" .)