Box of transparent mineral (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'of transparent mineral' is the wordplay.
'of' indicates a hidden word.
'SPAR' is hidden within 'transparent mineral'.
(Other definitions for spar that I've seen before include "Sticks" , "Box; mineral; pole" , "dispute" , "Length of wood" , "Raps with wooden beam" .)