Unpleasantness (9)
I believe the answer is:
nastiness
'unpleasantness' is the definition.
(nastiness is a kind of unpleasantness)
This is all the clue.
(Other definitions for nastiness that I've seen before include "repugnance" , "ill humour" , "Unpleasant behaviour" , "hostility" , "Pollution" .)