Put an element to the tail of a collection

concat does not add an element to the tail of a collection, nor does it concatenate two collections.

concat returns a seq made of the concatenation of two other seqs. The original type of the collections from which seqs may be inferred are lost for the return type of concat.

Now, clojure collections have different properties one must know about in order to write efficient code, that's why there isn't a universal function available in core to concatenate collections of any kind together. To the contrary, list and vectors do have "natural insertion positions" which conj knows, and does what is right for the kind of collection.

This is a very small addendum to @amalloy's answer in order to address OP's request for a function that always adds to the tail of whatever kind of collection. This is an alternative to (concat coll [x]). Just create a vector version of the original collection:

(defn conj*
  [s x]
  (conj (vec s) x))

Caveats:

If you started with a lazy sequence, you've now destroyed the laziness--i.e. the output is not lazy. This may be either a good thing or a bad thing, depending on your needs.

There's some cost to creating the vector. If you need to call this function a lot, and you find (e.g. by benchmarking with Criterium) that this cost is significant for your purposes, then follow the other answers' advice to try to use vectors in the first place.

Some types of collections can add cheaply to the front (lists, seqs), while others can add cheaply to the back (vectors, queues, kinda-sorta lazy-seqs). Rather than using concat, if possible you should arrange to be working with one of those types (vector is most common) and just conj to it: (conj [1 2 3] 4) yields [1 2 3 4], while (conj '(1 2 3) 4) yields (4 1 2 3).