Left behind, a group one day (9)
I believe the answer is:
abandoned
'left behind' is the definition.
(abandoning is a kind of leaving behind)
'a group one day' is the wordplay.
'group' becomes 'band' (band is a kind of group).
'day' becomes 'd'.
'a'+'band'+'one'+'d'='ABANDONED'
(Other definitions for abandoned that I've seen before include "lacking in self-control" , "Gave up - left - being completely irresponsible" , "Left in the lurch, forsaken" , "Left desolate or empty" , "Deserted; uninhibited" .)