Chorus splits regarding key (4,4)

I believe the answer is:

part song

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'chorus' is the definition.
Both the answer and definition are singular nouns.
Perhaps they are linked in a way I don't understand?

'splits regarding key' is the wordplay.
'splits' becomes 'parts' ('part' can be a synonym of 'split').
'regarding' becomes 'on' (on or regarding a particular topic).
'key' becomes 'g' (musical key).
'parts'+'on'+'g'='PART-SONG'

Can you help me to learn more?

(Other definitions for part song that I've seen before include "Madrigal, for example" , "number" , "It's sung" , "Piece for chorus" , "lay?" .)

I've seen this clue in The Times.

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