Merciful post-war prime minister, say? (7)
I believe the answer is:
clement
I believe this clue is a double definition.
'merciful' is the first definition.
(associated in meaning)
'post-war prime minister say?' is the second definition.
(PM Clement Attlee is an example)
(Other definitions for clement that I've seen before include "Mild or balmy" , "Mild; gentle; merciful" , "9, the chef and MP" , "mercy" , "opponent for Winston" .)