Crossword Genius

Extra hearing transposes the limits of ultrasound (8)

Ross

I believe the answer is:

addition

'extra' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for addition that I've seen before include "Supplement" , "Tally, tot" , "Increment" , "Mathematical procedure" , "Summing up process?" .)

I've seen this clue in The Guardian.
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