The next reign after Louis XVI? (6)
I believe the answer is:
terror
'reign after louis xvi?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'the next' is the wordplay.
I cannot really see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
The remaining letters 'error' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for terror that I've seen before include "Young rascal -- horror" , "Utter dread" , "consternation" , "Overwhelming feeling of fear" , "State of great fright" .)