Hydroacoustic device (5)
I believe the answer is:
sonar
'hydroacoustic device' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sonar that I've seen before include "Underwater navigation device" , "Underwater detection system" , "Method of echolocation" , "Underwater detector" , "Device for detecting objects under water with sound waves" .)