Something toxic with Plaid Cymru, and Liz Trussed? (7)
I believe the answer is:
mercury
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for mercury that I've seen before include "In Roman myth, the messenger of the gods" , "First rock from the Sun - Hg" , "Quicksilver" , "Planet - liquid metal" , "Metal in thermometer or barometer" .)