He clasps the crag (line 1)
The eagle must hold on tightly to the rock because the cliff is so steep. The word "clasp" could make you think of a hair clasp or the clasp that holds a stack of papers together. The eagle doesn't have to struggle to stay upright.
he stands (line 3)
The entire first stanza builds to this anti-climax. Compared to words like "clasps," "ring'd," and "azure," the final word "stands" isn't very exciting. Think of someone standing watch or standing guard.
The wrinkled sea beneath him crawls (line 4)
We see a contrast between old ("wrinkled") and young ("crawls") in this line. Everything in the world below is either too old or too young. But the eagle, in Goldilocks terms, is just right. The eagle is a creature in the prime of its strength and vitality.