Little bird -- one about to go under an enormous boat, that would be unfortunate (7)
I believe the answer is:
titanic
But I can't explain it at all! Could you help me understand?
(Other definitions for titanic that I've seen before include "Having huge strength - ship that famously sunk" , "Giant" , "Film success (1997) - ship disaster (1912)" , "Of great size, like a sunken ship" , "Exceptionally powerful" .)