Crossword Genius

Early form of sonar used to detect submarines (5)

Ross

I believe the answer is:

asdic

(Other definitions for asdic that I've seen before include "It once sounded" , "It sounds" , "Early echo-sounder; a disc (anag.)" , "Sound means of detection" , "Submarine detection device" .)

I've seen this clue in The Guardian.
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