Crossword Genius

Unbeatable opponent first to serve with the first seed returning (7)

Ross

I believe the answer is:

nemesis

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'unbeatable opponent' is the definition.
(I've seen this in another clue)

I don't understand the rest of the clue.

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(Other definitions for nemesis that I've seen before include "Nee Miss (anag.)" , "Vengeful justice" , "Come-uppance" , "Bane" , "enemy" .)

I've seen this clue in The Independent.
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