Navigational aid both ways (5)
I believe the answer is:
radar
I believe this clue is a double definition.
'navigational aid' is the first definition.
(I've seen this before)
'both ways' is the second definition.
(Other definitions for radar that I've seen before include "Remote range sensor" , "Method for detecting distant planes, ships etc" , "Electronic detection system" , "Palindromic detection system" , "System of detection by echoes" .)