Once, a prime minister's garden (4)

I believe the answer is:

eden

I'm a little stuck... Click here to teach me more about this clue!

'garden' is the definition.
(Garden of Eden in the Bible)

I don't understand the remainder of the clue.

(Other definitions for eden that I've seen before include "5,27 [PRIME MINISTER]" , "Foreign Secretary" , "(Biblical) paradise" , "Perfect place" , "Old Testament garden" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app