How team-stealer entered argument (4,5)
I believe the answer is:
took sides
'entered argument' is the definition.
The definition and answer can be both to do with fundamental existence as well as being past participle verbs.
Perhaps there's a link between them I don't understand?
'how team-stealer' is the wordplay.
I cannot really see how this works, but
'team' could be 'side' (both can mean a sports team) and 'side' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for took sides that I've seen before include "was biased" , "Photographed both teams" .)