Radar's kin (5)
I believe the answer is:
sonar
'radar's kin' 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 "What a near-blind creature uses" , "Detection system at sea" , "System to locate underwater objects" , "Underwater detection system" , "Device for detecting objects used in arson" .)