Carrying very minute drops from impure liquid, runs towards source (7)

I believe the answer is:

upriver

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

I don't understand the rest of the clue.

(Other definitions for upriver that I've seen before include "From mouth to source" , "Nearer to the source of the stream" , "Away from the coast" , "bucking current trend" , "Against the flow?" .)

I've seen this clue in The Times.

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