Important — ex-PM (5)
I believe the answer is:
major
I believe this clue is a double definition.
'important' is the first definition.
(major can mean of great importance)
'ex-pm' is the second definition.
(former prime minister John Major)
(Other definitions for major that I've seen before include "Large" , "Principal" , "Army officer immediately junior to a lieutenant colonel" , "Main course" , "Important; old Animal Farm boar" .)