Foreign Office taking back dirty books (6)

I believe the answer is:

folios

'books' is the definition.

'foreign office taking back dirty' is the wordplay.
'foreign office' becomes 'fo'.
'taking' is a charade indicator (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'.
'fo'+'lios'='FOLIOS'

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