The fish that sinks beneath the waves? (7)
I believe the answer is:
torpedo
'the fish' is the definition.
(torpedo is a kind of fish)
I can't explain the remainder of the clue.
Can you help me to learn more?
(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" .)