Echolocation device (5)
I believe the answer is:
sonar
'echolocation 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 "Locating device" , "Echo-sounding equipment" , "Underwater detection system" , "Underwater echo sounder" , "Method of echolocation" .)