What does the cryptic crossword clue "I'm telling the truth they're the upper crust (4)" mean?

The definition is "the upper crust". The wordplay uses a charade (building blocks). NO (synonym="NO") + BS (synonym="BS") = NOBS The answer is NOBS.

I'm telling the truth they're the upper crust (4)

Independent Cryptic #11683 | 15 Across | 2024-03-21 flower