Abbr. in a Beatles title (4)
I believe the answer is:
ussr
(Other definitions for ussr that I've seen before include "Country established in1922 and dissolved in 1991, in short" , "The Beatles? song 'Back in the . . . .'" , "Country established in 1922 and dissolved in 1991, in short" , "In brief, former Soviet federation" , "Former federation of Russial and other states" .)