Underwater detector (5)
I believe the answer is:
sonar
'underwater detector' 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 "System for locating underwater objects" , "Undersea sensing system" , "Echo sounder - arson (anag)" , "What a near-blind creature uses" , "Device for detecting objects used in arson" .)