Flat slabs of fired clay (5)
I believe the answer is:
tiles
'flat slabs of fired clay' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tiles that I've seen before include "Thin flat slabs, for roofing perhaps" , "Glazed rectangles used to cover walls or floors" , "Thin flat slabs of fired clay" , "Small oblong slabs used in roofing" , "Roofing materials" .)