"A representation of an abstract or spiritual meaning through concrete or material forms; figurative treatment of one subject under the guise of another. A symbolical narrative."
An allegory is not quite a metaphor. A metaphor compares or associates one idea or thing with another, in order to give a new understanding: "Hope is the thing with feathers," "My heart was like a singing bird" and so forth. An allegory is more extended. It uses one thing (a physical thing) as a stand-in, to represent a non-physical idea. Still, it can be hard to draw the line between "extended metaphor" and "allegory."
Allegory is a powerful tool because it can take an idea that is nebulous and hammer it into your brain, give it a shape and a bright colour.
The Handbook points to the Pilgrim's Progress as the most traditional allegory in English -- where you have actual characters and physical places representing attributes or abstract concepts -- the Slough of Despond, Mr. Greatheart, Faithful, etc. My beloved Faery Queene is another long allegory.
A good way to begin to write an allegory might be to write a "dream poem." What the dreamer encounters in the dream represents something different.
Here is an allegorical poem I really like. It was written by George Herbert in the 17th century, so I'm pretty sure it's in the public domain by now!
Love bade me welcome, yet my soul drew back,
Guilty of dust and sin,
But quickeyed Love, observing me grow slack
From my first entrance in
Drew nearer to me, sweetly questioning
If I lacked any thing.
"A guest," I answered, "worthy to be here--"
Love said, "You shall be he."
"I the unkind, the ungrateful? Ah, my Dear,
I cannot look on thee."
Love took my hand and smiling did reply
"Who made the eyes but I?"
"Truth, Lord--but I have marred them. Let my shame
Go where it doth deserve."
"And know you not," says Love, "who bore the blame?"
"My Dear, then I will serve."
"You must sit down," says Love, "and taste my meat."
So I did sit and eat.
Another one might be "The Sick Rose" by William Blake (18th cent, I believe):
O Rose, thou art sick.
The invisible Worm
That crawls in the Night
In the howling Storm
Hath found out thy bed
Of Crimson joy
And his dark Secret Love
Doth thy Life destroy.
Another amazing one is "The Question" by May Swenson. READ IT.
Do you have any allegory to share? Either a favourite poem, or something you're working on? I will post a few of mine as comments -- you are welcome to do so as well :)