Thoroughly depraved person returns in secret to Russia (6)
I believe the answer is:
rotter
'thoroughly depraved person' is the definition.
(a rotter is a depraved or rotten individual)
'returns in secret to russia' is the wordplay.
'returns' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'rettor' is hidden within 'secret to russia'.
'rettor' reversed gives 'ROTTER'.
(Other definitions for rotter that I've seen before include "Absolute bounder" , "Despicable person" , "Mean and unkind person" , "Cad, stinker" , "One not to be honoured" .)