Former prime minister's garden (4)
I believe the answer is:
eden
I believe this clue is a double definition.
'former prime minister's' is the first definition.
(I've seen this before)
'garden' is the second definition.
(I've seen this before)
(Other definitions for eden that I've seen before include "The perfect garden in need" , "Place of complete bliss" , "Old PM" , "Adam and Eve home" , "Old Testament garden" .)