The problem is that your use of let is wrong.
let works like this:
(let [identifier (expr)])
So your example should be something like this:
(let [s (Scanner. "a b c")]
(exprs))
You can only use the lexical bindings made with let within the scope of let (the opening and closing parens). Let just creates a set of lexical bindings. I use def for making a global binding and lets for binding something I want only in the scope of the let as it keeps things clean. They both have their uses.
NOTE: (Class.) is the same as (new Class), it’s just syntactic sugar.