... posted up during unpleasantness (4)
I believe the answer is:
sent
'posted' is the definition.
('send' can be a synonym of 'post')
'up during unpleasantness' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'during' says the answer is hidden in the clue.
'tnes' is hidden within 'unpleasantness'.
'tnes' backwards is 'SENT'.
(Other definitions for sent that I've seen before include "Transmitted" , "Remitted" , "Shipped" , "Posted" , "Caused to go somewhere" .)