Mythical sea monster (6)
I believe the answer is:
kraken
'mythical sea monster' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for kraken that I've seen before include "Mythical sea monster (Norway)" , "Mythical sea-monster off Norway's coast" , "Sea monster that sleepeth (Tennyson)" , "A legendary sea monster of Norwegian myth" , "Legendary monster of Norwegian seas" .)