Number in G & S, on reflection (4)

I believe the answer is:

song

I'm a little stuck... Click here to teach me more about this clue!

'number' is the definition.
(musical number)

'g s on reflection' is the wordplay.
'g s on' anagrams to 'SONG'.
But, I'm not sure how the anagram is indicated.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for song that I've seen before include "Ditty" , "You work best when on this" , "Vocal composition" , "See 17" , "Musical rendition" .)

I've seen this clue in The Independent.

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