Leviathan (3,7)
'leviathan' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for giant that I've seen before include "Extremely large (person)" , "Large black and white mammal" , "Monster" , "Colossus" , "Huge person" .)
(Other definitions for sea monster that I've seen before include "fabulous swimmer" , "dangerous creature in the main" , "A danger to mariners" .)