Sea anemone (5)
I believe the answer is:
polyp
'sea anemone' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for polyp that I've seen before include "Small unwanted growth in the body" , "Sea anemone, perhaps" , "Minute reef-building creature" , "Sea anemone, for example" , "Small growth on a mucous membrane" .)