Tennis player refuses naps altogether (8)

I believe the answer is:

entirely

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'altogether' is the definition.
('entirely' can be a synonym of 'altogether')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for entirely that I've seen before include "Without reservation" , "Wholly and completely" , "Wholly, totally" , "'Fully, wholly (8)'" , "Wholly, completly" .)

I've seen this clue in the Irish Times.

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