Drain requires disinfectant to remove source of blockage (5)

I believe the answer is:

leach

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'drain' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for leach that I've seen before include "Percolate - be removed by percolating liquid" , "Drain away by percolation" , "Filtrate" , "Make liquid percolate through a solid" , "Remove (soluble material) by percolation" .)

I've seen this clue in The Times.

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