Mythical sea monster (6)
I believe the answer is:
kraken
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mythical sea monster</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)