Joined Labour Party (5,4)
I believe the answer is:
chain gang
'joined labour party' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'labour party' is the wordplay.
'labour' becomes 'chain' (I am not sure about this - if you are sure you should believe this answer much more).
'party' becomes 'gang' (both can mean a group of people).
'chain'+'gang'='CHAIN-GANG'
Can you help me to learn more?
(Other definitions for chain gang that I've seen before include "Convicts forced to work" , "Linked prisoners" , "penal group" , "Convicts' work group" , "Shackled workers?" .)