Reef composition (5)
I believe the answer is:
coral
'reef composition' 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 "Marine polyps, often forming reefs" , "Reef-forming sea life" , "Carol (anag.)" , "Creature building reef" , "Orangey-pink" .)