Crossword Genius

Croon without fluctuation (4)

Ross

I believe the answer is:

sing

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

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for sing that I've seen before include "Warble" , "pipe" , "Confess" , "Chorus" , "Turn informer" .)

I've seen this clue in The Hindu.
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