That somewhat rigorous Russian! (4)
I believe the answer is:
igor
'russian' is the definition.
(common Russian name)
'that somewhat rigorous' is the wordplay.
'that somewhat' indicates the answer is hidden within the clue.
'IGOR' is hidden in the letters of 'rigorous'.
(Other definitions for igor that I've seen before include "-- Stravinsky, composer" , "Card game" , "Prince of Russian opera" , "Maybe 8 [SIKORSKY]" , "Prince of grand opera" .)