PM’s left without fellow ministers’ support, ultimately (9)
I believe the answer is:
gladstone
'pm's' is the definition.
(William Gladstone)
I don't understand the remainder of the clue.
(Other definitions for gladstone that I've seen before include "Old English P.M. or a doctor's bag" , "Politician" , "Former English PM or kind of bag" , "19th century PM" , "Tangled so like hinged case" .)