Succumbing to tired, empty feeling that engulfs vocal quartet (7,3)

I believe the answer is:

falling for

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'succumbing to tired empty feeling' is the definition.
I can't tell whether this defines the answer.

'engulfs vocal quartet' is the wordplay.
'engulfs' becomes 'falling' (I can't explain this - if you can you should give a lot more credence to this answer).
'vocal' shows a homophone (sound like).
'quartet' becomes 'four' (synonyms).
'four' is a homophone of 'for'.
'falling'+'for'='FALLING FOR'

'that' is the link.

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I've seen this clue in the Sydney Morning Herald.

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