Crossword Genius

Song performed with not so much feeling (6)

Ross

I believe the answer is:

number

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'song' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for number that I've seen before include "Mathematical unit" , "One possibly" , "Count; amount" , "One may be irrational or negative" , "Digit, cipher" .)

I've seen this clue in The Telegraph.
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