Couple in slavery (4)
I believe the answer is:
yoke
This clue is a double definition.
'couple' is the first definition.
(I know that couple can be written as yoke)
'slavery' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for yoke that I've seen before include "Bond of servitude" , "Frame joining draught oxen at the necks" , "Old harness or oppression" , "Burdensome restraint or oppression" , "Oppressive rule" .)