Abandoned side (4)
I believe the answer is:
left
This clue is a double definition.
'abandoned' is the first definition.
(leave can mean to abandon)
'side' is the second definition.
(as opposed to right)
(Other definitions for left that I've seen before include "Gone away - remaining" , "Sailed from" , "political group" , "Departed, went away" , "inclined towards socialism" .)