One who's done for (5)
I believe the answer is:
goner
'one who's done for' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for goner that I've seen before include "A has-been in distress" , "One past helping" , "person about to depart" , "One doomed to die" , "One without hope" .)