For singer, maybe, arrange costume — it's to be altered (3,2,5)

I believe the answer is:

set to music

'for singer maybe arrange' is the definition.
(as in to arrange music for a singer)

'costume it's to be altered' is the wordplay.
'to be altered' is an anagram indicator.
'costume'+'its'='costumeits'
'costumeits' with letters rearranged gives 'SET TO MUSIC'.

I've seen this clue in The Guardian.

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