Definite force seizing city (7)
I believe the answer is:
precise
'definite' is the definition.
('precise' can be a synonym of 'definite')
'force seizing city' is the wordplay.
'force' becomes 'prise' (I've seen this before**).
'seizing' means one lot of letters goes inside another.
'city' becomes 'ec' (the City of London postcode).
'prise' going around 'ec' is 'PRECISE'.
(Other definitions for precise that I've seen before include "'Exact, accurate (7)'" , "Express" , "with accuracy" , "Exact like recipes" , "Recipes (anag.)" .)