Conduct diplomacy including managed source of secrecy (8)
I believe the answer is:
transact
'conduct' is the definition.
(thesaurus)
'diplomacy including managed source of secrecy' is the wordplay.
'diplomacy' becomes 'tact' (I've seen this before).
'including' means one lot of letters goes inside another.
'managed' becomes 'ran' (I've seen this before**).
'source of' suggests taking the first letters.
The initial letter of 'secrecy' is 's'.
'ran'+'s'='rans'
'tact' enclosing 'rans' is 'TRANSACT'.
(Other definitions for transact that I've seen before include "Buy or sell" , "Perform or carry out business" , "Conduct trade" , "handle" , "Conduct business, negotiate" .)