Roofing square (4)
I believe the answer is:
tile
'roofing square' 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 "9 for the roof?" , "Flat piece of ceramic" , "Roofing slab" , "Hat" , "Small slab of baked clay for covering walls, roofs etc" .)