The most common and most likely answer to this crossword clue is the 4 plus 2 letter word LIVE IN
I've seen this crossword clue in The Guardian
L
I
V
E
I
N
Buddy explains!
The straight part
We know that "As an au pair would" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as an au pair would = live in.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LIVE IN mean?
LIVE IN adjective- Living on the premises, usually said of a household employee.