Expert to pass agreement (8)
I believe the answer is:
protocol
'agreement' is the definition.
(I know that protocol is a type of treaty)
'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 "Formal code" , "Draft treaty" , "Code of conduct, etiquette" , "Formalities" , "Code of behaviour" .)