Opposed to lassos but fighting a losing battle? (7,3,5)

I believe the answer is:

against the ropes

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'fighting a losing battle?' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'opposed to lassos' is the wordplay.
'opposed to' becomes 'against' (I've seen this in another clue).
'lassos' becomes 'theropes' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'against'+'theropes'='AGAINST THE ROPES'

'but' is the link.

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I've seen this clue in the Sydney Morning Herald.

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