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