Joined Labour Party? (5,4)
I believe the answer is:
chain gang
'joined labour party?' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'labour party?' is the wordplay.
'labour' becomes 'chain' (I can't justify this - if you can you should give a lot more credence to this answer).
'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?" .)