Opposed to poetry, mostly by us (6)

I believe the answer is:

versus

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'opposed to' is the definition.
(I've seen this before)

'poetry mostly by us' is the wordplay.
'poetry' becomes 'vers' (I've seen this before).
'mostly' means to remove the last letter.
'by us' becomes 'sus' (I am not sure about this - if you are sure you should believe this answer much more).
'vers' with its final letter taken away is 'ver'.
'ver'+'sus'='VERSUS'

Can you help me to learn more?

(Other definitions for versus that I've seen before include "v. (sport)" , "Against (opponent)" , "Set against" , "Preposition meaning against, as in contest" , "Against in games" .)

I've seen this clue in The Telegraph.

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