Defence building protecting city with our open area (9)
I believe the answer is:
forecourt
'open area' is the definition.
'forecourt' can be an answer for 'area' (forecourt is a kind of area). I'm not certain of the 'open' bit.
'defence building protecting city with our' is the wordplay.
'defence building' becomes 'fort' (I can't explain this - if you can you should believe this answer much more).
'protecting' indicates putting letters inside (inserted letters are held or protected).
'city' becomes 'EC' (the City of London postcode).
'with' says to put letters next to each other.
'ec'+'our'='ecour'
'fort' placed around 'ecour' is 'FORECOURT'.
Can you help me to learn more?
(Other definitions for forecourt that I've seen before include "Open space" , "Where cars may be sold" , "Area of filling station" , "vehicle may have pulled up here" .)