Long time Conservative occupying US city office (6)
I believe the answer is:
agency
'office' is the definition.
(I know that office can be written as agency)
'long time conservative occupying us city' is the wordplay.
'long time' becomes 'age' (an age is a period of time).
'conservative' becomes 'C' (UK political abbreviation).
'occupying' means one lot of letters goes inside another (inserted letters occupy or enter the word).
'us city' becomes 'NY' (short for New York).
'c' placed within 'ny' is 'ncy'.
'age'+'ncy'='AGENCY'
(Other definitions for agency that I've seen before include "Organisation providing a particular service" , "Travel ..., advertising..., employment" , "Government office" , "Function of one who acts for another" , "Business that serves other businesses" .)