Finished in a group, so left alone (9)
I believe the answer is:
abandoned
'left alone' is the definition.
(to abandon someone is to leave them on their own)
'finished in a group' is the wordplay.
'finished' becomes 'done' (similar in meaning).
'in' means one lot of letters goes inside another.
'group' becomes 'band' (band can mean a group of people).
'a'+'band'='aband'
'done' placed into 'aband' is 'ABANDONED'.
'so' is the link.
(Other definitions for abandoned that I've seen before include "Profligate - gave up - left" , "Given up?" , "Derelict, deserted" , "marooned" , "Gave up - left - being completely irresponsible" .)