Advanced info over case of counterspy operation (6)
I believe the answer is:
agency
'operation' is the definition.
(thesaurus)
'advanced info over case of counterspy' is the wordplay.
'advanced' becomes 'a'.
'info' becomes 'gen' (informal term for information).
'over' means one lot of letters go next to another.
'case of' says to hollow out the word (remove centre letters) (outsides of).
'counterspy' with its middle removed is 'cy'.
'a'+'gen'+'cy'='AGENCY'
(Other definitions for agency that I've seen before include "See 24 Across" , "Business organisation" , "Through which something is accomplished" , "Business that acts for actors" , "Travel --; dating --" .)