A single thing
A whispered wing
A china clink
A ribboned trinket
A single ship
Upon the sea
A sunny day
A silent sillouette
The first letter of the alphabet and the first letter of my legal name.
In addition to being a letter, "A" is a word as well and it is this word rather than the letter that you most likely clicked to get here, as it is an extremely common word. It means a single thing, an indefinite article I think it's called. Essential cartilage for the body of the english language.