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' says to put 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" .)