Be against including it or just the reverse (8)
I believe the answer is:
opposite
'the reverse' is the definition.
(synonyms)
'be against including it' is the wordplay.
'be against' becomes 'oppose' (synonyms).
'including' indicates putting letters inside.
'oppose' placed around 'it' is 'OPPOSITE'.
'or just' acts as a link.
(Other definitions for opposite that I've seen before include "Contrary or face-to-face" , "Totally different; facing" , "completely different?" , "Facing - reverse" , "Rival" .)