Fight against it being contained on the other side (8)
I believe the answer is:
opposite
'on the other side' is the definition.
(I've seen this before)
'fight against it being contained' is the wordplay.
'fight against' becomes 'oppose' (I've seen this before).
'being contained' indicates putting letters inside.
'oppose' enclosing 'it' is 'OPPOSITE'.
(Other definitions for opposite that I've seen before include "Corresponding" , "Facing on the other side" , "Directly facing each other" , "Oppose it, being faced against" , "Diametrically contrary" .)