Marine formation (5)
I believe the answer is:
coral
'marine formation' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for coral that I've seen before include "Pinkish-orange colour" , "certain islands" , "Creature building reef" , "Shade of pink" , "(Massed skeletons of) tiny invertebrate" .)