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.