Formula ready for use city area gets brought in (6)
I believe the answer is:
recipe
'formula' is the definition.
(synonyms)
'ready for use city area gets brought in' is the wordplay.
'ready for use' becomes 'ripe'.
'city area' becomes 'ec' (EC is a London postcode area).
'gets brought in' is an insertion indicator.
'ripe' going around 'ec' is 'RECIPE'.
(Other definitions for recipe that I've seen before include "Pierce (anag) - directions" , "Directions for preparing food, etc" , "Directions for cooking a dish" , "Formula for making something" , "Set of instructions for preparing a dish" .)