As before, by the way, is opposed to it (7)
I believe the answer is:
against
'opposed to it' is the definition.
'against' can be an answer for 'opposed' (I've seen this in another clue). I'm not sure about the 'to it' bit.
'as before by the way' is the wordplay.
'as before' becomes 'again' (I am not sure about this - if you are sure you should believe this answer much more).
'by the way' becomes 'st' (I've seen this in other clues).
'again'+'st'='AGAINST'
'is' is the link.
Can you help me to learn more?
(Other definitions for against that I've seen before include "Not in favour of" , "In opposition to" , "close by" , "In competition with" , "Facing" .)