Domino piece (4)
I believe the answer is:
tile
'domino piece' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tile that I've seen before include "Glazed piece of earthenware as for lining walls" , "Hat" , "Thin slab of fired clay" , "9 for the roof?" , "Flat piece of ceramic" .)