Federation that dissolved in 1991 (4)

I believe the answer is:

ussr

(Other definitions for ussr that I've seen before include "Country established in 1922 and dissolved in 1991, in short" , "Former Soviet Union" , "Former association of communist republics" , "Former union of socialist republics" , "The Beatles? song 'Back in the . . . .'" .)

I've seen this clue in The Washington Post.

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