Shogun's missing son is Flood? (4)

I believe the answer is:

gush

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'is flood?' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

(Other definitions for gush that I've seen before include "Outburst" , "Spout like geyser" , "Flow unceasingly" , "Spout like oil-well" , "Pour out - appear overly effusive" .)

I've seen this clue in the Irish Times and the Irish Times (Crosaire).

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