Shut crystal box (4)
I believe the answer is:
spar
This clue might be a double definition.
'shut crystal' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'box' is the second definition.
(sparring is a kind of boxing)
Can you help me to learn more?
(Other definitions for spar that I've seen before include "Box (lightly) - fight (verbally)" , "Pole used to support sails or rigging" , "Argue" , "Box in training - mineral" , "possibly rigged" .)