In central Madrid, crowds stunned to find swamp (5)

I believe the answer is:

drown

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'swamp' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for drown that I've seen before include "Swamp" , "Completely conceal" , "Flood; kill" , "Kill by submersion" , "Die by inhaling liquid" .)

I've seen this clue in The Guardian.

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