Terrible Russian supporting head of dynastic seat (5)
I believe the answer is:
divan
'seat' is the definition.
(divan is a kind of seat)
'terrible russian supporting head of dynastic' is the wordplay.
'terrible russian' becomes 'Ivan' (Ivan the Terrible).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'head of' suggests taking the first letters.
The initial letter of 'dynastic' is 'd'.
'ivan' put after 'd' is 'DIVAN'.
(Other definitions for divan that I've seen before include "resting place" , "Backless seat; bed" , "Sideless, backless couch" , "Sort of bed" , "Piece of furniture" .)