What does the cryptic crossword clue "As an au pair would at No 54 with a German (4,2)" mean?

The wordplay uses a charade (building blocks). LIV (synonym="54 in Roman numerals") + EIN (synonym="German “a”") = LIVEIN The answer is LIVEIN.

As an au pair would at No 54 with a German (4,2)

Guardian Cryptic #28282 | 25 Across | 2020-11-04 flower