Many a part of London is 'on-Sea'! (7)
I believe the answer is:
clacton
'many a part of london is on-sea' is the definition.
I can't judge whether this definition defines the answer.
'many a part of london is on' is the wordplay.
'many a' becomes 'cl' (I can't explain this - if you can you should believe this answer much more).
'part of london' becomes 'acton' (I've seen this before).
'is on' says to put letters next to each other.
'cl'+'acton'='CLACTON'
Can you help me to learn more?
(Other definitions for clacton that I've seen before include "Essex town" , "Resort near Colchester" , "seaside town" , "Seaside resort in Essex" .)