CIA's files reviewed and ultimately marked confidential (10)
I believe the answer is:
classified
'confidential' is the definition.
(both can mean top secret)
'cia's files reviewed and ultimately marked' is the wordplay.
'reviewed' indicates an anagram (letters reviewed or revised).
'and' says to put letters next to each other.
'ultimately' suggests the final letters.
The final letter of 'marked' is 'd'.
'cias'+'files'='ciasfiles'
'ciasfiles' with letters rearranged gives 'classifie'.
'classifie'+'d'='CLASSIFIED'
(Other definitions for classified that I've seen before include "Arranged in categories" , "Categorised; secret" , "to be kept secret" , "Ordered" .)