As an au pair would at No 54 with a German (4,2)
I believe the answer is:
live in
'as an au pair would' is the definition.
(an au pair lives in the household that hires her)
'no 54 with a german' is the wordplay.
'no 54' becomes 'liv' (54 in Roman numerals).
'with' says to put letters next to each other.
'a german' becomes 'ein' ('a' in German).
'liv'+'ein'='LIVE-IN'
'at' is the link.
(Other definitions for live in that I've seen before include "Cohabiting" , "Reside at one's place of employment" , "Resident at one's place of work" , "(Of employee or lover) resident" .)