Beheaded terrible ruler in leading position (3)
I believe the answer is:
van
'leading position' is the definition.
(term for the front of a group eg a military group)
'beheaded terrible ruler' is the wordplay.
'beheaded' means to remove the first letter (the head of the word is removed).
'terrible ruler' becomes 'ivan' (tsar of Russia Ivan the Terrible).
'ivan' with its initial letter removed is 'VAN'.
'in' is the link.
(Other definitions for van that I've seen before include "Covered wagon" , "Delivery vehicle" , "trailer" , "sometimes seen beside dyke" , "Front of an advancing army" .)