Opposed once more to secret ends (7)
I believe the answer is:
against
'opposed' is the definition.
(I've seen this in another clue)
'once more to secret ends' is the wordplay.
'once more' becomes 'again' (synonyms).
'to' is a charade indicator (letters next to each other).
'ends' says to hollow out the word (remove centre letters).
'secret' with its centre taken out is 'st'.
'again'+'st'='AGAINST'
(Other definitions for against that I've seen before include "Not supporting" , "In competition with" , "Opposed" , "Not in favour of" , "Placed in direct contact with" .)