Tessellation piece (4)
I believe the answer is:
tile
'tessellation 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 "Roof slabs" , "Flat, thin slab to cover surface" , "Thin slab of fired clay" , "Scrabble piece" , "Thin rectangular slab to cover surfaces" .)