A crystal box (4)
I believe the answer is:
spar
This clue might be a double definition.
'a crystal' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'box' is the second definition.
(spar can mean to box)
Can you help me to learn more?
(Other definitions for spar that I've seen before include "Mast - boom - gaff" , "Box - girder" , "Train at boxing; mineral" , "Close friend" , "Box; various minerals" .)