Such a fish as sinks beneath the waves? (7)
I believe the answer is:
torpedo
I believe this clue is a double definition.
'such a fish' is the first definition.
(torpedo is a kind of fish)
'as sinks beneath the waves?' is the second definition.
(underwater missile)
(Other definitions for torpedo that I've seen before include "Trooped (anag.)" , "fish you want" , "(Of a project) cause to sink without trace" , "Trooped off with undersea missile" , "Missile form a submarine" .)