Be against including it or just the converse (8)
I believe the answer is:
opposite
'the converse' is the definition.
(I've seen this before)
'be against including it' is the wordplay.
'be against' becomes 'oppose' (synonyms).
'including' means one lot of letters goes inside another.
'oppose' placed around 'it' is 'OPPOSITE'.
'or just' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for opposite that I've seen before include "Facing - reverse" , "Corresponding" , "across the road?" , "Contrary or face-to-face" , "Diametrically contrary" .)