Withdraw City journalist in the Home Counties (6)
I believe the answer is:
secede
'withdraw' is the definition.
(I've seen this before)
'city journalist in the home counties' is the wordplay.
'city' becomes 'ec' (the City of London postcode).
'journalist' becomes 'ed' (short for editor).
'in the' indicates putting letters inside.
'home counties' becomes 'se'.
'ec'+'ed'='eced'
'eced' put within 'se' is 'SECEDE'.
(Other definitions for secede that I've seen before include "Withdraw formally from membership or alliance" , "Withdraw (from organization)" , "Withdraw from, say organisation" , "'Withdraw formally, from a political body perhaps (6)'" , "Break away (politically)" .)