Opposed to party prepared to return remedies (9)
I believe the answer is:
antidotes
'remedies' is the definition.
(antidote is a kind of remedy)
'opposed to party prepared to return' is the wordplay.
'opposed to' becomes 'anti' (I've seen this before).
'party' becomes 'do' (do is a kind of party).
'prepared' becomes 'set' (I've seen this before).
'to return' says the letters should be written backwards.
'set' written backwards gives 'tes'.
'anti'+'do'+'tes'='ANTIDOTES'
(Other definitions for antidotes that I've seen before include "Cures for poisons" , "Counteractions to poisons" , "Remedies" .)