A race supporting Diana's verbal onslaught (8)
I believe the answer is:
diatribe
'verbal onslaught' is the definition.
(I've seen this before)
'a race supporting diana's' is the wordplay.
'race' becomes 'tribe' (I've seen this before).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'diana' becomes 'di' (Di is short for Diana).
'a'+'tribe'='atribe'
'atribe' put after 'di' is 'DIATRIBE'.
(Other definitions for diatribe that I've seen before include "Bitter words" , "provoking verbal onslaught" , "Abusive harangue" , "Torrent of abuse" , "Long speech" .)