Extravagantly spend start of Sunday in Chelsea (5)

I believe the answer is:

blues

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'start of sunday in chelsea' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for blues that I've seen before include "Australian redheads" , "Jazz classic (2,5,5) - See 9" , "US music style" , "Conservatives" , "Melancholic music of black American folk origin" .)

I've seen this clue in The Guardian.

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