US partnership in Civil Service, a cause of much amusement (7)
I believe the answer is:
cahoots
'us partnership' is the definition.
'cahoots' can be an answer for 'partnership' (I've seen this in another clue). I'm unsure of the 'us' bit.
'in civil service a cause of much amusement' is the wordplay.
'in' is an insertion indicator.
'civil service' becomes 'CS' (abbreviation).
'a cause of much amusement' becomes 'ahoot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cs' enclosing 'ahoot' is 'CAHOOTS'.
Can you help me to learn more?
(Other definitions for cahoots that I've seen before include "Collusion" , "League" , "(In) conspiracy" , "In - means colluding" , "In such agreement" .)