Joined daughter supporting team (5)
I believe the answer is:
sided
'joined' is the definition.
The answer and definition 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?
'daughter supporting team' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'team' becomes 'side' (both can mean a sports team).
'd' put after 'side' is 'SIDED'.
Can you help me to learn more?
(Other definitions for sided that I've seen before include "Allied oneself (with one party)" , "Showed solidarity (with)" , "Took part with person in a dispute" .)