Ready to assimilate City formula (6)
I believe the answer is:
recipe
'formula' is the definition.
('recipe' can be a synonym of 'formula')
'ready to assimilate city' is the wordplay.
'ready' becomes 'ripe' ('ripe' can be similar in meaning to 'ready'**).
'to assimilate' means one lot of letters goes inside another.
'city' becomes 'ec' (the City of London postcode).
'ripe' placed around 'ec' is 'RECIPE'.
(Other definitions for recipe that I've seen before include "method for achieving success" , "Kitchen instructions" , "Kitchen directions" , "Instructions on how to cook" , "Directions for preparing food, etc" .)