Gigantic, colossal (7)
I believe the answer is:
titanic
This clue is a double definition.
'gigantic' is the first definition.
(I've seen this before)
'colossal' is the second definition.
(I've seen this before)
(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" .)