In part of East End that's housing for the toffs? (7)
I believe the answer is:
popular
'in' is the definition.
('in' can mean popular or trendy)
'part of east end that's housing for the toffs?' is the wordplay.
'part of east end' becomes 'Poplar' (Poplar is an area of east London).
'that's housing' means one lot of letters goes inside another (inserted letters are housed by others).
'for the toffs?' becomes 'U' (upper class, as in U and non-U).
'poplar' enclosing 'u' is 'POPULAR'.
(Other definitions for popular that I've seen before include "Fashionable" , "In demand" , "Widely liked" , "The people's" , "In general favour" .)