Surface for Painting in Oils
Puzzle 18 of National Crossword Day Hard Pack
Question is: Surface for Painting in Oils
and the answer is: CANVAS
Word Meaning:
CANVAS — A type of coarse cloth, woven from hemp (traditionally) or from cotton and polyesters, useful for making sails, tents, and overcoats or as a surface for paintings.
Definitions from Wiktionary.