Something toxic seen when peeling layers from lethal onion? (8)
I believe the answer is:
thallium
'onion?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for thallium that I've seen before include "Metallic element used in rat poison" , "Metallic element, Tl" .)