Soon to overlook Liberal, about to overlook messenger (8)
I believe the answer is:
emissary
'messenger' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for emissary that I've seen before include "Person sent to negotiate" , "Someone sent on a mission" , "Diplomat sent on special task" , "Delegate" , "Government messenger" .)