Boundary Foreign Office applied to old city (4)
I believe the answer is:
four
'boundary' is the definition.
(type of score in cricket)
'foreign office applied to old city' is the wordplay.
'foreign office' becomes 'fo'.
'applied to' means one lot of letters go next to another.
'old city' becomes 'ur' (ancient city state).
'fo'+'ur'='FOUR'
(Other definitions for four that I've seen before include "Cube root of 64" , "Bridge quorum" , "Perfect square" , "Number of players in a doubles game" , "How many Horsemen of the Apocalypse?" .)