Crossword Genius

Early form of sonar developed by the British between the two World Wars (5)

Ross

I believe the answer is:

asdic

(Other definitions for asdic that I've seen before include "Early echo-sounder; a disc (anag.)" , "Ultrasonic detector" , "old detection method" , "It once sounded" , "means to find hidden problems" .)

I've seen this clue in The Mirror.
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