The Clerk is a man who works in a small shop.
After Homer had decided to drive Red Barclay's truck to Atlanta, he and Bart needed to drive 2200 miles in 10 hours. Homer decided to stop in a small shop to buy some pep pills so he could drive all the night. The clerk gave him a bottle of Stimu-Crank.
Homer grabbed it and ate almost all of the pills. The clerk was shocked and told him that Homer should not eat that many pills at once. Homer replied that he would "balance it out" with some sleeping pills, grabbed another bottle and ate many pills from it. Then he ran out of the shop, ready to drive all the night.