It forms reefs in warm seas (5)
I believe the answer is:
coral
'it forms reefs in warm seas' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for coral that I've seen before include "Calcareous skeletal remains forming reefs" , "Marine substance, often forms reef" , "Basis of reefs" , "Basic material of South Sea islands" , "Pink growth which forms reefs and islands" .)