Rummikub piece (4)
I believe the answer is:
tile
'rummikub piece' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tile that I've seen before include "Mah-jong piece" , "Ceramic rectangle" , "Flat, thin slab to cover surface" , "Roofing or flooring item" , "Small slab of baked clay for covering walls, roofs etc" .)