What does the cryptic crossword clue "Boron insulator for bum (7)" mean?

The wordplay uses a charade (building blocks). B (synonym="boron") + LAGGER (synonym="insulator") = BLAGGER The answer is BLAGGER.

Boron insulator for bum (7)

Guardian Cryptic #29462 | 15 Across | 2024-08-15 flower