A shore thing' is an idiom that refers to something that is certain or guaranteed. The term comes from the idea that the shore is always there, so it is a constant and reliable presence. The crossword solution for this term is SHELL, which refers to the hard outer covering of some animals, such as turtles and clams. This is a metaphorical connection to the idea of something being a sure thing, as the SHELL is a protective barrier that ensures the safety of the animal inside.