Terrible scare grips a Roman leader (6)
I believe the answer is:
caesar
'roman leader' is the definition.
(Julius Caesar)
'terrible scare grips a' is the wordplay.
'terrible' is an anagram indicator.
'grips' means one lot of letters goes inside another.
'scare' with letters rearranged gives 'caesr'.
'caesr' going around 'a' is 'CAESAR'.
(Other definitions for caesar that I've seen before include "See 13 Across" , "Augustus or Hadrian, say" , "Eg, Augustus, Julius" , "Roman chief like Julius" , "Old Roman emperor like Julius" .)