Russian physiologist (6)
I believe the answer is:
pavlov
'russian physiologist' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pavlov that I've seen before include "Russian psychologist" , "Russian who studied the conditioned reflex using dogs" , "conditioner" , "Russian who studied animals' reflexes" , "Ivan --, Russian studier of conditioned reflexes" .)