Facts retained by Foreign Office (4)
I believe the answer is:
info
'facts' is the definition.
(I've seen this in another clue)
'retained by foreign office' is the wordplay.
'retained by' becomes 'in' (I can't justify this - if you can you should give a lot more credence to this answer).
'foreign office' becomes 'FO' (abbreviation).
'in'+'fo'='INFO'
Can you help me to learn more?
(Other definitions for info that I've seen before include "Data received and understood (abbr)" , "Intelligence (abbr)" , "Knowledge (colloq.)" , "Latest" , "Knowledge provided" .)