Sub's tracking device (5)
I believe the answer is:
sonar
'sub's tracking device' 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 "Underwater detection device" , "Device for detecting objects used in arson" , "echo device" , "Method of echolocation" , "Underwater navigation device" .)