Posted about rising obscenity, it's grating (8)
I believe the answer is:
strident
'it's grating' is the definition.
(I've seen this before)
'posted about rising obscenity' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'about' indicates putting letters inside.
'rising' is a reversal indicator (going up in a down clue).
'obscenity' becomes 'dirt' (I've seen this before).
'dirt' reversed gives 'trid'.
'sent' enclosing 'trid' is 'STRIDENT'.
(Other definitions for strident that I've seen before include "(Of a voice) loud and grating" , "giving views forcefully" , "Unpleasantly rough" , "Clamorous" , "Roaring" .)