Finished among a group left behind (9)
I believe the answer is:
abandoned
'left behind' is the definition.
(abandoning is a kind of leaving behind)
'finished among a group' is the wordplay.
'finished' becomes 'done' (both can mean completed).
'among' is an insertion indicator (inserted letters are placed among others).
'group' becomes 'band' (band can mean a group of people).
'a'+'band'='aband'
'done' going within 'aband' is 'ABANDONED'.
(Other definitions for abandoned that I've seen before include "Left - dissolute" , "Gave up - left - being completely irresponsible" , "Given up?" , "Wild" , "Deserted; uninhibited" .)