Opening of typed post still happening (5)
I believe the answer is:
event
'happening' is the definition.
(I've seen this before)
'opening of typed post still' is the wordplay.
'opening of' says to take the initial letters.
'post' is a charade indicator (letters next to each other) (post- can mean after).
'still' becomes 'even' (synonyms).
The first letter of 'typed' is 't'.
't' after 'even' is 'EVENT'.
(Other definitions for event that I've seen before include "contest in programme" , "Something going on" , "Something happening" , "case" , "match" .)