Former federation (4)
I believe the answer is:
'former federation' 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 association of communist republics" , "Country established in1922 and dissolved in 1991, in short" , "Former federation of Russial and other states" , "Former union of socialist republics" , "Former Soviet Union" .)