Opening mail (4)
I believe the answer is:
post
I believe this is a double definition.
'opening' is the first definition.
(thesaurus)
'mail' is the second definition.
(synonyms)
(Other definitions for post that I've seen before include "Spot the mail" , "Pole - position" , "Appointment (with the Royal Mail?)" , "(Holiday) communication" , "ob" .)