Mythical sea creature (7)
I believe the answer is:
mermaid
'mythical sea creature' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mermaid that I've seen before include "Woman with tail of fish" , "Mythical sea dweller" , "Mythical sea creature" , "'Half woman, half fish (7)'" , "London theatre?" .)