Communist state to 1991 (4)
I believe the answer is:
(Other definitions for ussr that I've seen before include "Country established in1922 and dissolved in 1991, in short" , "Former association of communist republics" , "The Beatles? song 'Back in the . . . .'" , "Country established in 1922 and dissolved in 1991, in short" , "Former Soviet Union" .)