Forsook, left behind (9)
I believe the answer is:
abandoned
I believe this clue is a double definition.
'forsook' is the first definition.
('abandon' can be a synonym of 'forsake')
'left behind' is the second definition.
(abandoning is a kind of leaving behind)
(Other definitions for abandoned that I've seen before include "Left forlorn" , "Crazy!" , "Unrestrained" , "Deserted; uninhibited" , "Left - dissolute" .)