Stone only containing element (4)
I believe the answer is:
neon
'element' is the definition.
(neon is a kind of element)
'stone only containing' is the wordplay.
'containing' indicates the answer is hidden within the clue.
'NEON' is hidden within 'stone only'.
(Other definitions for neon that I've seen before include "One of the six inert gases" , "None like bright city light" , "Gaseous element, in advertising signs, say" , "Element 10, used in lights" , "Gas used in fluorescent lights" .)