Pottery material (4)
'pottery material' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for clay that I've seen before include "Moulding material" , "Material used by potter" , "Raw material for pottery" , "slip" , "Brick/pottery substance" .)
(Other definitions for terracotta that I've seen before include "brownish-red colour" , "Ornamental building material" , "casting" , "reddish colour" , "Pottery" .)