Poison (tl) Used in Christie's the Pale Horse
This page is the CodyCross answer for the clue below from Greece, Group 672, Puzzle 5 (Art Galleries).
Question is: Poison (tl) Used in Christie's the Pale Horse
and the answer is: THALLIUM
Word Meaning:
THALLIUM — A metallic chemical element (symbol Tl) with atomic number 81: a gray post-transition metal that discolors when exposed to air.
Definitions from Wiktionary.