Exclusion (9)
'exclusion' is the definition.
(I know that ostracism is a type of exclusion)
This is all the clue.
(Other definitions for ostracism that I've seen before include "Exclusion from a group or society" , "being sent to Coventry" , "Banishment" , "isolation" , "Shunning" .)
(Other definitions for exception that I've seen before include "Objectors may take it" , "what's against the rules" , "Something that does not follow the rule" , "Something not conforming to a rule" , "Something not following a rule" .)
'exclusion' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for omission that I've seen before include "Negligence" , "It may create space" , "Something left out or not done" , "cut?" , "task not done" .)