In any given Discworld book I often find a word or two I don't know. Here's such a passage from Men at Arms:
"Bad case of Flameless Gripe," said Lady Ramkin. "Hope we've got it in time–"
The dragon ripped its way out of the sack and looked around for something to incinerate. Everyone tried to get out of the way.
Then its eyes crossed, and it hiccuped.
The limestone tablet pinged off the opposite wall.
They leapt for such cover as was provided by a water-trough and a pile of clinkers.
The dragon hiccuped again, and looked puzzled.
Then it exploded.
I like the line about such cover, although I don't know what clinkers are. Wikipedia describes clinker as waste from industrial processes, although it uses the term as a mass noun, which isn't right for a "pile of clinkers." But it also links to clinker bricks as the original use of the term, which sound about right for this passage Clinkers in that sense are (the page tells me) a messed-up form of normal bricks, produced by firing wet clay bricks at too hot a temperature. They "have a blackened appearance, and they are often misshapen or split," which sounds about right for dingy Ankh-Mopork.
So, it seems to be a pile of messed-up almost-bricks they're using for cover.