She’s back in the USSR (3)
I believe the answer is:
sue
'she's' is the definition.
'back in the ussr' is the wordplay.
'back' says the letters should be written backwards.
'in' indicates a hidden word.
'eus' is hidden within 'the ussr'.
'eus' backwards is 'SUE'.
(Other definitions for sue that I've seen before include "Begin legal proceedings against" , "Apply" , "Appeal" , "Proceed legally against (someone)" , "Take court action" .)