Secret family decree in advance (11)
I believe the answer is:
clandestine
'secret' is the definition.
(synonyms)
'family decree in advance' is the wordplay.
'family' becomes 'clan' (clan can mean a large family).
'decree in advance' becomes 'destine' (I can't justify this - if you can you should give a lot more credence to this answer).
'clan'+'destine'='CLANDESTINE'
Can you help me to learn more?
(Other definitions for clandestine that I've seen before include "Concealed" , "Send a client (anag) - covert" , "Done secretively" , "through the backdoor" , "Private" .)