Ex-UN member (4)
I believe the answer is:
ussr
'ex-un member' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ussr that I've seen before include "Former soviet states" , "Country established in1922 and dissolved in 1991, in short" , "Former union of socialist republics" , "The Beatles? song 'Back in the . . . .'" , "Country established in 1922 and dissolved in 1991, in short" .)