Unpleasantness for SAS sent in mistakenly (9)
I believe the answer is:
nastiness
'unpleasantness' is the definition.
(I know that nastiness is a type of unpleasantness)
'sas sent in mistakenly' is the wordplay.
'mistakenly' indicates anagramming the letters.
'sas'+'sent'+'in'='sassentin'
'sassentin' is an anagram of 'NASTINESS'.
'for' acts as a link.
(Other definitions for nastiness that I've seen before include "hostility" , "Malevolence" , "Pollution" , "Unpleasant behaviour" , "Repugnant behaviour" .)