Mineral still has poisonous element (6)
I believe the answer is:
killas
'mineral still' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'poisonous element' is the wordplay.
'poisonous' becomes 'kill' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'element' becomes 'As' (As is an example).
'kill'+'as'='KILLAS'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for killas that I've seen before include "Slate" , "Hard rock" .)