Direction-finder that may be boxed (7)
I believe the answer is:
compass
This clue is a double definition.
'direction-finder' is the first definition.
'may be boxed' is the second definition.
(I've seen this before)
'that' is the link.
(Other definitions for compass that I've seen before include "Navigation instrument" , "Navigational instrument" , "Moss cap (anag.)" , "Range - vessel's navigating instrument" , "Range, area" .)