Former Communist state (4)
I believe the answer is:
'former communist state' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ussr that I've seen before include "Country established in 1922 and dissolved in 1991, in short" , "Former union of socialist republics" , "Former federation of Russial and other states" , "The Beatles? song 'Back in the . . . .'" , "Former association of communist republics" .)