Somewhat sombre, a king (Theban) kept demanding too much money (8,3,4)

I believe the answer is:

breaking the bank

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'too much money' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'somewhat sombre a king theban kept' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'BREAKING THE BANK' is hidden in the letters of 'sombre a king theban kept'.

'demanding' is the link.

Can you help me to learn more?

I've seen this clue in The Everyman puzzle in The Observer.

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