He often assisted insane scientist dig for missing heads (4)
I believe the answer is:
'he' is the definition.
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for igor that I've seen before include "Stravinsky's first name" , "Stravinsky, perhaps" , "Prince --, opera by Borodin" , "Prince in Borodin's opera" , "Card game" .)