Rehearse Nicholson piece containing example of this deadly material (6)
I believe the answer is:
poison
This could be a double definition.
'rehearse nicholson piece containing example of this' is the first definition.
The definition suggests a singular noun which matches the answer.
'deadly material' is the second definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe you can see a link between them that I can't see?
Can you help me to learn more?
(Other definitions for poison that I've seen before include "Damaging stuff - drink?" , "Meat for someone else" , "Bane" , "Toxic substance" , "[ADDER] 's defence" .)