Major supports party line (6)
I believe the answer is:
domain
'line' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'major supports party' is the wordplay.
'major' becomes 'main' (both can mean important).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'party' becomes 'do' (do can informally mean a party).
'main' after 'do' is 'DOMAIN'.
Can you help me to learn more?
(Other definitions for domain that I've seen before include "Area under someone's control, field of activity" , "Realm or range of influence" , "estate" , "Scope of knowledge - owned land" , "Field of thought or activity" .)