Detection system (5)
'detection system' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for radar that I've seen before include "System that detects and locates distant objects" , "Pulsed location-finder" , "beams" , "A help in locating" , "Locator of plane" .)
'detection system' is the definition.
(sonar is a system that detects using sound waves)
This is all the clue.
(Other definitions for sonar that I've seen before include "location device" , "Detection system at sea" , "Method of echolocation" , "Device for detecting objects used in arson" , "Underwater detection system" .)