What does the cryptic crossword clue "Presumably she wouldn't have voted for Brexit (6)" mean?

The wordplay uses a charade (building blocks). EU (synonym="EU") + NICE (synonym="NICE") = EUNICE The answer is EUNICE.

Presumably she wouldn't have voted for Brexit (6)

Guardian Cryptic #29074 | 24 Across | 2023-05-19 flower