Rival groups, leaderless, demand to enter thus (4,3,2)
I believe the answer is:
them and us
'rival groups' is the definition.
'leaderless demand to enter thus' is the wordplay.
'leaderless' means to remove the first letter.
'to enter' is an insertion indicator.
'demand' with its first letter taken away is 'emand'.
'emand' put into 'thus' is 'THEM AND US'.
(Other definitions for them and us that I've seen before include "Opposing groups" , "adversarial situation" , "Two camps" , "workers' view of management?" , "Opposites" .)