The fish that sinks beneath the waves? (7)

I believe the answer is:

torpedo

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'the fish' is the definition.
(torpedo is a kind of fish)

I can't explain the remainder of the clue.

(Other definitions for torpedo that I've seen before include "attack" , "Submarine weaponry" , "Trooped off with undersea missile" , "Trooped (anag.)" , "Self-propelled underwater explosive device" .)

I've seen this clue in The Sun.

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