Expert to pass diplomatic code (8)
I believe the answer is:
protocol
'diplomatic code' is the definition.
(I've seen this in another clue)
'expert to pass' is the wordplay.
'expert' becomes 'pro' (short for professional).
'pass' becomes 'col' (col is a term for a mountain pass).
'pro'+'to'+'col'='PROTOCOL'
(Other definitions for protocol that I've seen before include "Draft treaty" , "Good form" , "Etiquette with regard to rank or status" , "Correct procedure" , "Code of correct conduct" .)