It detects movement back and forth (5)

I believe the answer is:

radar

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'back and forth' is the definition.

I don't understand how the remainder of the clue works.

(Other definitions for radar that I've seen before include "Method of detection" , "Early warning system" , "Kind of gun" , "Electronic detection system" , "Remote detection system" .)

I've seen this clue in The Times.

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