What does the cryptic crossword clue "Something toxic with Plaid Cymru, and Liz Trussed? (7)" mean?

The wordplay uses an anagram. anagram of ER (synonym="“Liz”, the late queen") + CYMRU (synonym="CYMRU") = MERCURY The answer is MERCURY.

Something toxic with Plaid Cymru, and Liz Trussed? (7)

Guardian Cryptic #29426 | 7 Across | 2024-07-04 flower