What does the cryptic crossword clue "Extremist partly responsible for dreadful tragedy (5)" mean?

The definition is "Extremist". The wordplay uses a hidden word. Hidden in "extremist partly responsible for dreadfUL TRAgedy" The answer is ULTRA.

Extremist partly responsible for dreadful tragedy (5)

Guardian Cryptic #29072 | 20 Across | 2023-05-17 flower