Upset later by a disloyal act (8)
I believe the answer is:
betrayal
'disloyal act' is the definition.
'upset later by a' is the wordplay.
'upset' indicates anagramming the letters.
'later'+'by'+'a'='laterbya'
'laterbya' anagrammed gives 'BETRAYAL'.
(Other definitions for betrayal that I've seen before include "Act of treachery" , "sale" , "Selling" , "Treacherous act" , "The crime of Judas" .)