Crossword Genius

By depriving an ex-swimmer of sleep (3)

Ross

I believe the answer is:

per

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'by' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for per that I've seen before include "According to" , "Preposition indicating by means of" , "Preposition meaning for each" , "For each or every" .)

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