Foreign Office taking back dirty books (6)
I believe the answer is:
Here is my best explanation:
'books' is the definition.
'foreign office taking back dirty' is the subsidiary indication.
'foreign office' becomes 'fo'.
'taking' says to put letters next to each other.
'back' says the letters should be written in reverse.
'dirty' becomes 'soil' ('soil' can be a synonym of 'dirty').
'soil' reversed gives 'lios'.