Ready to receive city's instructions (6)
I believe the answer is:
recipe
'instructions' is the definition.
(I've seen this before)
'ready to receive city's' is the wordplay.
'ready' becomes 'ripe' ('ripe' can be similar in meaning to 'ready'**).
'to receive' means one lot of letters goes inside another.
'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" , "Instruction for cooking" , "Directions for making food etc" , "Directions for preparing food, etc" , "Set of instructions for preparing a dish" .)