Empire collapsed with Republican leader (7)
I believe the answer is:
premier
'leader' is the definition.
(premier is a kind of leader)
'empire collapsed with republican' is the wordplay.
'collapsed with' is an anagram indicator.
'republican' becomes 'r' (abbreviation).
'empire' mixed up with 'r' can make 'PREMIER'.
(Other definitions for premier that I've seen before include "Which is having precedence" , "man who's in charge" , "First (minister)" , "Foremost" , "Earliest" .)