Left single day after joining a team (9)
I believe the answer is:
abandoned
'left' is the definition.
(abandoning is a kind of leaving)
'single day after joining a team' is the wordplay.
'single' becomes 'one' (a single thing - only one).
'day' becomes 'd' (abbreviation for day).
'after joining' says to put letters next to each other.
'team' becomes 'band' (both can mean a group of people).
'one'+'d'='oned'
'a'+'band'='aband'
'oned' after 'aband' is 'ABANDONED'.
(Other definitions for abandoned that I've seen before include "Deserted - shameless" , "marooned" , "lacking in self-control" , "Given up?" , "Forsook, left behind" .)