Leaders of the Soviets assassinated Russian ruler (4)
I believe the answer is:
tsar
'ruler' is the definition.
(historical term for Russian king)
'leaders of the soviets assassinated russian' is the wordplay.
'leaders of' indicates taking the first letters.
The initial letters of 'the soviets assassinated russian' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Formal rulings on Islamic law; was aft (anag.)" , "Old Russian ruler" , "Autocrat" , "Absolute ruler" , "Supreme authority" .)