Swinging joints (6)
I believe the answer is:
hinges
'swinging joints' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hinges that I've seen before include "Depends on - like door?" , "Flexible joints" , "Movable joints between lid and box, say" , "A door swings on them" , "Doors swing on them" .)