Just a language that isn't yet used widely in production. I remember when Python was like that, there is even a relevant xkcd strip: https://xkcd.com/353/
What are you talking about? Walmart has built their receipt processing in Clojure. Apple uses it (afaik for payments processing). Cisco has built their entire security platform in Clojure - security.cisco.com. Funding Circle has built their peer-to-peer landing platform in Clojure. Nubank - the largest independent digital bank in the world and sixth-largest bank in Brazil been using Clojure extensively. There are many other companies very actively using Clojure. Pandora, CircleCI, Pitch, Guaranteed Rate, etc. It's even used at NASA.
It's a the third most popular JVM language after Java and Kotlin, and the most popular alt-js PL (if you don't consider Typescript as alt-js). It's the most popular language among PLs with a strong FP emphasis - it is more popular than Haskell, Elm, Idris, OCaml, Erlang, Elixir, F#, Purescript, and (recently) Scala.
Clojure is very ripe for the prime-time. The ecosystem is really nice. A lot more nicer than most other languages. It is an extremely productive tool. But of course skeptics be like: "but it's dyyying ...", "it ain't popular ...", "but all those parentheses ...", "it's a cult ...", etc.
Clojure is used in production a lot, a big majority of users report using it for work. There's been significant shift from from the enthusiast-dominated community days of 10 years ago.
Gosh, I remember when JAVA was like that!