Physiologist Pavlov (4)
I believe the answer is:
ivan
'physiologist pavlov' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ivan that I've seen before include "- - the Terrible (Czar)" , "First tsar of Russia" , "Russian, say" , "terrible tyrant" , "Vain (anag) - Tsar of Russia" .)