Cluster is the oldest cultivated hop variety in American brewing, traced to plants grown by colonial-era farmers with parentage that was never formally documented the way modern USDA and university releases are. It was the dominant American hop for well over a century, used almost universally before Cascade and the rest of the modern breeding era arrived in the 1970s, and it's still grown today even though its commercial role has shrunk considerably since.
At 5.5-8.5% alpha acid, Cluster functions mainly as a bittering hop, with a black currant and earthy aroma that reads as old-fashioned next to the citrus and tropical fruit character of newer American varieties — it was never bred for a distinctive aroma the way modern proprietary hops are, since it predates the entire concept of breeding hops specifically for flavor marketing. Cluster's alpha is moderate and workmanlike rather than sharp, so a bittering charge shouldn't be sized as if it behaves like the modern hops that eventually displaced it — the IBU calculator will show the real difference.
Nugget and Galena are the closest substitutes for Cluster's bittering role, both offering comparable or higher alpha acid with their own distinct but similarly unglamorous character. Cluster remains most useful in historical-recipe brewing — pre-Prohibition American lagers and early 20th-century ale styles — where its presence is closer to historical accuracy than aromatic preference.