Prime Minister once great leader? (5,4)
I believe the answer is:
north star
'great leader?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'prime minister once' is the wordplay.
'prime minister' becomes 'north' (I've seen this before).
'once' becomes 'star' (I am not sure about this - if you are sure you should believe this answer much more).
'north'+'star'='NORTH STAR'
Can you help me to learn more?
(Other definitions for north star that I've seen before include "feature of Ursa Minor" , "Navigational aid" , "Polaris" , "guiding light" , "nickname of US State" .)