Each post requires start of tour in part of London (4,3)
I believe the answer is:
east end
'part of london' is the definition.
(I've seen this before)
'each post requires start of tour in' is the wordplay.
'each' becomes 'ea' (abbreviation).
'post' becomes 'send' (synonyms).
'requires' says to put letters next to each other.
'start of' says to take the initial letters.
'in' indicates putting letters inside.
The initial letter of 'tour' is 't'.
'ea'+'send'='easend'
'easend' placed around 't' is 'EAST END'.
(Other definitions for east end that I've seen before include "where plates are for standing on?" , "Cockneyland" , "Part of London first to see rising sun" , "ockney London" , "7 Down in London" .)