“Goodbye” upsets Isadora no end (5)
I believe the answer is:
adios
'goodbye' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for adios that I've seen before include "Goodbye (20 across)" , "Goodbye (Spanish)" , "So long, amigo" , "Au revoir, amigo!" , "Bye-bye (amigo)" .)