One visiting source of holy river, supposed to be announced (5,5)

I believe the answer is:

house guest

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'one visiting source' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'holy river supposed to be announced' is the wordplay.
'holy river' becomes 'house' (I can't explain this - if you can you should give a lot more credence to this answer).
'supposed' becomes 'guessed' ('guess' can be a synonym of 'suppose').
'to be announced' shows a homophone (sound like).
'guessed' sounds like 'guest'.
'house'+'guest'='HOUSE GUEST'

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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