Skag is a rough slang word for heroin, the illegal drug. You will hear it in older street talk, crime stories, and some films more than in careful news writing, where people just say heroin. It is informal and grim in tone — not a joke word, and not something you use in class or in polite conversation. Like many drug slang words, it can sound dated depending on where you are; younger speakers may not use it at all. There is no extra “clean” meaning in this sense: it names the drug.