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