What does the cryptic crossword clue "18? Peace is truncated by one that’s sacked Gaul (8)" mean?

The wordplay uses an anagram. anagram of CAL (synonym="CAL") + GAUL (synonym="GAUL") + I (synonym="one") = CALIGULA The answer is CALIGULA.

18? Peace is truncated by one that’s sacked Gaul (8)

Guardian Cryptic #29481 | 21 Across | 2024-09-06 flower