Arsenic, for example — a noted conversation stopper? (5,5)
I believe the answer is:
heavy metal
'arsenic for example' is the definition.
(arsenic is a heavy metal)
I can't explain the remainder of the clue.
(Other definitions for heavy metal that I've seen before include "Lead, say" , "Rock music (big guns?)" , "Loud form of rock" , "hard rock" , "Loud rock music" .)