Unpleasantness involving Sean's tins, crushed (9)
I believe the answer is:
nastiness
'unpleasantness' is the definition.
(I know that nastiness is a type of unpleasantness)
'sean's tins crushed' is the wordplay.
'crushed' indicates an anagram.
'seans'+'tins'='seanstins'
'seanstins' anagrammed gives 'NASTINESS'.
'involving' is the link.
(Other definitions for nastiness that I've seen before include "ill humour" , "Repugnant behaviour" , "Malevolence" , "repugnance" , "Unpleasant behaviour" .)