Mythical sea creatures (8)
I believe the answer is:
mermaids
'mythical sea creatures' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mermaids that I've seen before include "They're half woman, half fish" , "Mythical creatures, half woman and half fish" , "They're half women, half fish" , "Imaginary sea-creatures" , "Mythical sea creatures" .)