Keep Foreign Office right (4)
I believe the answer is:
fort
'keep' is the definition.
(I've seen this in other clues)
'foreign office right' is the wordplay.
'foreign office' becomes 'fo'.
'right' becomes 'rt'.
'fo'+'rt'='FORT'
(Other definitions for fort that I've seen before include "Defensive stronghold" , "US gold reserves are at - Knox" , "Citadel" , "Defensive building" , "Keep" .)