Join duo (6)
I believe the answer is:
couple
I believe this clue is a double definition.
'join' is the first definition.
(coupling is a kind of joining)
'duo' is the second definition.
('couple' can be a synonym of 'duo')
(Other definitions for couple that I've seen before include "Two of a kind" , "item" , "Pair of attached people" , "Two - join up" , "The pair to join together" .)