Upset Abigail disregarding Georgia's excuse (5)
I believe the answer is:
alibi
'excuse' is the definition.
(alibiing is a kind of excusing)
'upset abigail disregarding georgia's' is the wordplay.
'upset' indicates an anagram.
'disregarding' indicates named letters should be taken away.
'georgia' becomes 'ga'.
'abigail' with 'ga' taken away is 'abiil'.
'abiil' with letters rearranged gives 'ALIBI'.
(Other definitions for alibi that I've seen before include "Pretext; court excuse" , "Legal term from the Latin 'elsewhere'" , "Cover story" , "Proof one was elsewhere when crime was committed" , "Defence against accusation - was seen elsewhere at the time" .)