Fashionable part of London occupied by the socially superior (7)
I believe the answer is:
popular
'fashionable' is the definition.
(similar in meaning)
'part of london occupied by the socially superior' is the wordplay.
'part of london' becomes 'poplar' (district of East London).
'occupied by' is an insertion indicator.
'the socially superior' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'poplar' placed around 'u' is 'POPULAR'.
Can you help me to learn more?
(Other definitions for popular that I've seen before include "Widely admired" , "In general favour" , "Commonly known" , "appealing to the public" , "Liked or admired by many" .)