"en" is the name of the 14th letter of the alphabet, so writing "nth" instead of "nth" must be the equivalent of writing "1st" instead of "first".
The answer looks too good to be wrong.
Thanks to all. In my view a lousy clue but there we are. In fact I find the latest generation of prize Xwords very unsatisfactory (O, my Araucaria!). The other week you could not use letters from other answers as a guide to unsolved clues as the answers were synonyms of the solutions. And how many solvers were familiar with Li Sao?