Gigantic, colossal (7)
I believe the answer is:
titanic
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">gigantic</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">colossal</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for titanic that I've seen before include "Gigantic like doomed ocean liner" , "Extremely large" , "Famously sunken ship" , "like Oceanus" , "Huge - disaster struck this ship" .)