One side remaining (4)
I believe the answer is:
left
I believe this clue is a double definition.
'one side' is the first definition.
(I've seen this before)
'remaining' is the second definition.
('left' can be a synonym of 'remaining')
(Other definitions for left that I've seen before include "quit" , "The side of socialism" , "Sailed from" , "Facing north, it's the side toward the west" , "walk-out?" .)