Expert on a branch of government (5)
I believe the answer is:
adept
'expert' is the definition.
(synonyms)
'on a branch of government' is the wordplay.
'on' says to put letters next to each other.
'branch of government' becomes 'dept' (I can't explain this - if you can you should believe this answer much more).
'a'+'dept' is 'ADEPT'.
Can you help me to learn more?
(Other definitions for adept that I've seen before include "Having the skill required" , "Thoroughly proficient" , "Deacon" , "Skillful" , "Proficient - expert" .)