Running water can act like acid! (4)

I believe the answer is:

burn

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

I can't explain the remainder of the clue.

(Other definitions for burn that I've seen before include "Catch fire; small stream" , "Set on fire" , "Scorch - stream" , "Scorch; brook" , "pain during exercise" .)

I've seen this clue in the Evening Standard.

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