Crossword Genius

Origin of ketchup, say (6)

Ross

I believe the answer is:

source

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'origin' is the definition.
(I know that origin can be written as source)

I don't understand how the remainder of the clue works.

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(Other definitions for source that I've seen before include "Find" , "Origin as of a river" , "From which something springs" , "Origin or starting point" , "Beginning to add" .)

I've seen this clue in The Irish Independent.
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