Echolocation (5)
(Other definitions for radar that I've seen before include "Locator that uses a microwave echo" , "Kind of gun" , "System for locating distant objects" , "Pulsed location-finder" , "Remote detection device" .)
'echolocation' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sonar that I've seen before include "Undersea sensing system" , "Underwater detector" , "Device for detecting objects used in arson" , "System for locating underwater objects" , "echo device" .)