Clap! Snap! The black crack!
Clap! Snap! The black crack! is the song sung by the goblins as they descend into Goblin-town after capturing Thorin, the dwarves, and Bilbo Baggins (but not Gandalf). The goblins kept time by the flap of their flat feet upon the stone floor of the passage. As they sang they brought out whips to force the dwarves and the hobbit to run as fast as they could.
The first stanza refers to the capture of Thorin's company in their "Front Porch", the seemingly small cave the company had taken shelter in while traversing the mountains. The second and third stanzas indicate that the captives will be turned into hard-working slaves of the goblins far within the mountains.
Clap! Snap! the black crack!
Grip, grab! Pinch, nab!
And down down to Goblin-town
You go, my lad!
Clash, crash! Crush, smash!
Hammer and tongs! Knocker and gongs!
Pound, pound, far underground!
Ho, ho! my lad!
Swish, smack! Whip crack!
Batter and beat! Yammer and bleat!
Work, work! Nor dare to shirk,
While Goblins quaff, and Goblins laugh,
Round and round far underground
Below, my lad!