Old PM without teeth leaving gallery (4)
I believe the answer is:
eden
'old pm' is the definition.
(Anthony Eden, UK prime minister 1955-1957)
'without teeth leaving gallery' is the wordplay.
'without teeth' becomes 'edentate' (I've seen this in another clue).
'leaving' suggests deleting specific letters.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'edentate' with 'tate' taken away is 'EDEN'.
(Other definitions for eden that I've seen before include "Delightful place" , "project in the south west" , "The first garden" , "PM 1955-56" , "Paradise - ex-prime minister" .)