US agency initially sent in to hide Republican immorality (6,7)
I believe the answer is:
secret service
'initially sent in to hide republican immorality' is the definition.
I can't tell whether this defines the answer.
'us agency' is the wordplay.
'us' becomes 'secret' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'agency' becomes 'service agency' (service agency is a kind of agency).
'secret'+'service'='SECRET SERVICE'
Can you help me to learn more?
(Other definitions for secret service that I've seen before include "Spies' department" , "MI5 or MI6 (informal)" , "Government agency" , "Government department involved in espionage" , "Hush-hush organisation" .)