Formula fully developed outside the City (6)
I believe the answer is:
recipe
'formula' is the definition.
('recipe' can be a synonym of 'formula')
'fully developed outside the city' is the wordplay.
'fully developed' becomes 'ripe' (I've seen this before).
'outside' indicates putting letters inside.
'the city' becomes 'ec' (the City of London postcode).
'ripe' enclosing 'ec' is 'RECIPE'.
(Other definitions for recipe that I've seen before include "Pierce (anag) - directions" , "Directions for preparing food, etc" , "Ingredient list" , "Chef's instructions" , "Cooking prescription" .)