Under my window, a clean rasping sound When the spade sinks into gravelly ground:
Someone outside is digging, using a spade, or shovel, and it's making a rasping sound when it cuts into the earth. Why rasping? Well, the shovel probably makes a horrible, grating sound when it enters the ground because the shovel is skimming against lots of tiny stones that make up the "gravelly ground."
"Clean" and "rasping" is another interesting pairing of words. "Rasping" and "gravelly" work well together because they're similar in meaning – rasping means coarse and gravel actually is coarse. "Clean" however, would much more likely make us think of something smooth and polished. So how can a sound be both coarse and smooth?
Regardless of the initial contradiction, a "clean rasping sound" is something we can make sense of. Think of a shovel being thrust quickly (and cleanly) into the ground. The sound is quick and clean, but you can detect the subtle rasp of the metal passing the tiny stones in the soil's gravel.
Now we have a more complete picture of our speaker writing at his window when he hears the sound of someone digging outside, because he's using imagery that engages both sight and sound. Thanks, Mr. Heaney.
My father digging. I look down
Oh, so it's the speaker's father who is doing the digging. Okay, good to know.
So far, there have been three different tools mentioned: the pen, the gun, and the spade.
When we step back, we see the speaker (the son) indoors, pen in hand, and outside, the father works digging at the rocky soil. Those are two very different activities.