Mythical sea creature (7)
I believe the answer is:
mermaid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mythical sea creature</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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?" .)