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Originally,
source
(excessively
loud
noise)
was
a
sub
(5,2)
Originally, source (excessively loud noise) was a sub (5,2)
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The definition is at the end of the clue.
How the clue works
Right, so was a sub is the definition there, as in stood in for someone. For the wordplay, take just the first letter of "source" giving you S (that's "originally"), then add TOO for "excessively", then DIN for "loud noise" - string those together and you get STOODIN, which splits out to STOOD IN.