Looking back and forth, finding apparatus (5)
I believe the answer is:
radar
I believe this is a double definition.
'looking back and forth' is the first definition.
'finding apparatus' is the second definition.
(Other definitions for radar that I've seen before include "Means of detecting and locating distant objects" , "Remote detection device" , "Range-finding and detection system" , "Electronic rangefinder" , "A help in locating" .)