Crossword Genius

Tiles built up a third-class cold painting well (7)

Ross

I believe the answer is:

ostraca

(Other definitions for ostraca that I've seen before include "Signs of exile" , "Tiles" .)

I've seen this clue in The Times.
Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.