New player joining team in collapse (7)
I believe the answer is:
subside
'collapse' is the definition.
(subside can mean to collapse or slump)
'new player joining team' is the wordplay.
'new player' becomes 'sub' (a substitute player).
'joining' is a charade indicator (letters next to each other).
'team' becomes 'side' (both can mean a sports team).
'sub'+'side'='SUBSIDE'
'in' is the link.
(Other definitions for subside that I've seen before include "decline" , "Go down in level" , "Collapse" , "Abate" , "Wear off or die down" .)