Under gravity, form striking carbon crystal (5)
I believe the answer is:
glass
'carbon crystal' is the definition.
'glass' can be an answer for 'crystal' (I've seen this in another clue). I am not sure about the 'carbon' bit.
'under gravity form striking' is the wordplay.
'under' says to put letters next to each other (in a down clue, some letters go below others).
'gravity' becomes 'g'.
'form striking' becomes 'lass' (I can't justify this - if you can you should believe this answer much more).
'g' next to 'lass' is 'GLASS'.
Can you help me to learn more?
(Other definitions for glass that I've seen before include "such as one might hold at party" , "Tumbler, mirror" , "Watch-face cover" , "barometer" , "Goblet" .)