It comes in to fight on the other side (8)
I believe the answer is:
opposite
'on the other side' is the definition.
(I've seen this in another clue)
'it comes in to fight' is the wordplay.
'comes in to' indicates putting letters inside.
'fight' becomes 'oppose' (synonyms).
'it' inserted within 'oppose' is 'OPPOSITE'.
(Other definitions for opposite that I've seen before include "completely different?" , "Contrary, antipode" , "Directly facing each other" , "Oppose it, being faced against" , "across the road?" .)