Thus a house is in one part of London (4)
I believe the answer is:
soho
'one part of london' is the definition.
(district of Westminster)
'thus a house' is the wordplay.
'thus' becomes 'so' (synonyms).
'a house' becomes 'ho' (abbreviation for house).
'so'+'ho'='SOHO'
'is in' acts as a link.
(Other definitions for soho that I've seen before include "fashionable neighbourhood (or neighborhood)" , "London district known for its nightlife" , "Exotic London district popular with tourists" , "London night life area" , "Central London entertainment area" .)