Private ceremony held in government department concerned with espionage (6,7)
I believe the answer is:
secret service
'espionage' is the definition.
(thesaurus)
'private ceremony' is the wordplay.
'private' becomes 'secret' (similar in meaning).
'ceremony' becomes 'service' (service is a kind of ceremony).
'secret'+'service'='SECRET SERVICE'
'held in government department concerned with' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for secret service that I've seen before include "Government agency" , "Government department involved in espionage" , "Spying agency" , "MI5 or MI6 (informal)" , "Spies' department" .)