Expert heading to officer convention (8)
I believe the answer is:
protocol
'convention' is the definition.
(thesaurus)
'expert heading to officer' is the wordplay.
'expert' becomes 'pro' (short for professional).
'heading' is a charade indicator (letters next to each other) (some letters go in front of or head others).
'officer' becomes 'col' (abbreviation for colonel, an army officer).
'pro'+'to'+'col'='PROTOCOL'
(Other definitions for protocol that I've seen before include "Poor clot involved in kind of status etiquette" , "Code of conduct, etiquette" , "Diplomatic etiquette or procedure" , "Diplomatic rules" , "Rules of conduct" .)