Building CIA operation is obsolete (7)
I believe the answer is:
archaic
'is obsolete' is the definition.
(I've seen this before)
'building cia operation' is the wordplay.
I cannot really see how this works, but
'building' could be 'arch' and 'arch' is found within the answer.
an anagram of 'cia' is 'aic' which is located in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for archaic that I've seen before include "Ancient or out of date" , "Behind the times" , "Ancient, no longer current" , "Extremely old" , "Belonging to former times like some words" .)