Church, otherwise a part of London (7)
I believe the answer is:
chelsea
'part of london' is the definition.
(Chelsea is an area of London)
'church otherwise a' is the wordplay.
'church' becomes 'ch' (abbreviation on maps etc).
'otherwise' becomes 'else' (synonyms).
'ch'+'else'+'a'='CHELSEA'
(Other definitions for chelsea that I've seen before include "London football team" , "Where you might get buns" , "Part of our capital" , "Kind of bun that's flat" , "Sort of bun" .)