Fool upset following the Parisian's language (5)
I believe the answer is:
latin
'language' is the definition.
(Latin is an example)
'fool upset following the parisian's' is the wordplay.
'fool' becomes 'nit' (I've seen this before).
'upset' says the letters should be written backwards.
'following' says to put letters next to each other.
'the parisian' becomes 'la' (French for the).
'nit' written backwards gives 'tin'.
'tin' after 'la' is 'LATIN'.
(Other definitions for latin that I've seen before include "a quarter of Parisians" , "Hispanic, for example" , "Romantic language" , "Parent language of French,Romanian" , "Cicero's tongue" .)