Oppressive burden (4)
I believe the answer is:
yoke
'oppressive burden' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for yoke that I've seen before include "Frame joining draught oxen at the necks" , "Coupling" , "Bond of servitude" , "Oppressive power" , "Burdensome restraint or oppression" .)