Reef matter (5)
I believe the answer is:
coral
'reef matter' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for coral that I've seen before include "Lobster or crab roe" , "Marine substance, often forms reefs" , "Hard substance on the seabed" , "Carol (anag.)" , "Pinkish-orange colour" .)