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 unsure of 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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'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" .)