Glassy substance — part of the name lacking (6)
I believe the answer is:
enamel
'glassy substance' is the definition.
(I've seen this before)
'part of the name lacking' is the wordplay.
'part of' indicates a hidden word.
'ENAMEL' is hidden in the letters of 'the name lacking'.
(Other definitions for enamel that I've seen before include "Coating for a tooth" , "Hard material" , "sort of paint" , "Hard white substance covering the crown of a tooth" , "canine protection?" .)