Reef material (5)
I believe the answer is:
coral
'reef material' is the definition.
(material making up a coral reef)
This is the entire clue.
(Other definitions for coral that I've seen before include "Basis of reefs" , "Tiny animal forming reef" , "Marine substance, often forming reefs" , "Basic material of South Sea islands" , "Hard substance on the seabed" .)