Crossword Genius

Encourage doctor to wipe out source of salmonella (4,2)

Ross

I believe the answer is:

urge on

'encourage' is the definition.
(I know that urge on is a more specific form of the action encourage)

I can't explain the remainder of the clue.

(Another definition for urge on that I've seen is " Encourage".)

I've seen this clue in The Guardian.
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