Undersea detection system (5)
I believe the answer is:
sonar
'undersea detection system' 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 "What a near-blind creature uses" , "Underwater detection device" , "Detection system at sea" , "Underwater detector" , "System to locate underwater objects" .)