The first liar upset lawsuit (5)
I believe the answer is:
trial
'lawsuit' is the definition.
(thesaurus)
'the first liar upset' is the wordplay.
'the first' becomes 't' (1st letter of 'the').
'upset' indicates an anagram.
'liar' with letters rearranged gives 'rial'.
't'+'rial'='TRIAL'
(Other definitions for trial that I've seen before include "Criminal court case" , "Court appearance on charge - it's an ordeal" , "Annoyance" , "Ordeal or lawsuit" , "Proceedings to determine guilt or innocence" .)