City welcomes MP with hat, one being firm (8)
I believe the answer is:
emphatic
'being firm' is the definition.
(I've seen this in another clue)
'city welcomes mp with hat one' is the wordplay.
'city' becomes 'EC' (the City of London postcode).
'welcomes' is an insertion indicator (in sense of 'invites in').
'with' says to put letters next to each other.
'one' becomes 'I' (Roman numeral).
'mp'+'hat'+'i'='mphati'
'ec' going around 'mphati' is 'EMPHATIC'.
(Other definitions for emphatic that I've seen before include "Categorical" , "For sure" , "Definite and clear" , "Direct" , "Forceful in expression" .)