Upset on account of border (9)
I believe the answer is:
crossover
'account of border' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'upset on' is the wordplay.
'upset' becomes 'cross' (I've seen this in another clue).
'on' becomes 'over' (I've seen this in other clues).
'cross'+'over'='CROSSOVER'
Can you help me to learn more?
(Other definitions for crossover that I've seen before include "mixing styles" , "in a mixture of styles" , "Rock opera perhaps" , "where traversal is made" , "affecting different functions" .)