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' means one lot of letters go next to another (some letters go in front of or head others).
'officer' becomes 'col' (short for colonel).
'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" .)