Big American tennis star is not ever ranked in leading positions (5)

I believe the answer is:

isner

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'big american tennis star' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'is not ever ranked in leading positions' is the wordplay.
'not ever ranked in leading positions' becomes 'ner' (I can't justify this - if you can you should give a lot more credence to this answer).
'is'+'ner'='ISNER'

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