Great number going for river plunge (7)

I believe the answer is:

immerse

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'plunge' is the definition.
(I know that plunge can be written as immerse)

I don't understand the rest of the clue.

(Other definitions for immerse that I've seen before include "Dip under the surface of a liquid completely" , "Submerge in water as once for baptism" , "Drown" , "Engage deeply" , "Dunk" .)

I've seen this clue in The Guardian.

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