Quartz that’s still pit-coal, oddly (6)
I believe the answer is:
silica
'quartz' is the definition.
(I've seen this in another clue)
'still pit-coal oddly' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
'still'+'pit'+'coal'='stillpitcoal'
The alternating letters of 'stillpitcoal' are 'SILICA'.
'that's' is the link.
(Other definitions for silica that I've seen before include "Common base of rock" , "Hard colourless compound from rocks" , "Quartz used in the manufacture of glass" , "Main constituent of sand" , "Constituent of sandstone and quartz" .)