It sinks below the waves (7)
I believe the answer is:
torpedo
'it sinks below the waves' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
Can you help me to learn more?
(Other definitions for torpedo that I've seen before include "Missile fired from underwater submarine" , "Self-propelled underwater explosive device" , "Naval missile" , "Underwater bomb" , "Naval weapon" .)