Erected huge house in part of London (4)
I believe the answer is:
soho
'part of london' is the definition.
(district of Westminster)
'erected huge house in' is the wordplay.
'erected' shows that the letters should be reversed in order (in a down clue, letters go up).
'huge' becomes 'os' (abbreviation for outsize).
'house' becomes 'ho' (abbreviation for house).
'in' indicates putting letters inside.
'os' enclosing 'ho' is 'ohos'.
'ohos' written backwards gives 'SOHO'.
(Other definitions for soho that I've seen before include "Call a large market for software" , "Central London or Manhattan district" , "W London area" , "Colourful neighbourhood in London's West End" , "fashionable neighbourhood (or neighborhood)" .)