The difference is that it is not possible to modify the value of `a` in the scope that it is bound too. Try as you want, but you won't be able to write an `inc` that modifies the value of `a` within that scope.
The declarative nature is that you're declaring that you want `a` to mean 10 within some scope. That's why you say "let `a` be 10 in current scope", that's your intention here, for `a` to be 10.
After you've declared that, it holds true no matter what.
Where as in the imperative style you say: put 10 at place `a`. This is variable assignment, there's a place which is refered too as `a` and inside that place you can set values and change them at will.
At any point you can instruct the language to change what value is at place `a`, there are no restrictions to the instructions you can give.
It's a bit of a oversimplification to claim that imperative programming is distinguished from declarative programming by assuming a lack of ordering in the latter.
Functional programming is not prevented from expressing order, rather it is less able to express random accidental order at the operational semantics level.
At the end of the day, it's very much about the computational model, imperative is based on Turing model, and Functional on the lambda calculus. Because the latter doesn't depend on a global mutable running state, it is said to be declarative, in the sense that what you see is what you get, you don't need to keep track of what the memory currently has to proceed to the next step.
That said, you're right that Clojure supports some forms of imperative programming as well, but let isn't one, your parent commenter pointing to Atom was more on point. That's what you'd need to do to get back a more imperative `let`:
(let [a (atom 10)
a (+ (swap! a inc) (swap! a inc))]
a)
Will now give you 23.