It's normal to be disturbed by such a serious breach (6,3)
I believe the answer is:
mortal sin
'breach' is the definition.
(thesaurus)
'it's normal to be disturbed' is the wordplay.
'to be disturbed' is an anagram indicator (the order of letters is disturbed).
'its'+'normal'='itsnormal'
'itsnormal' with letters rearranged gives 'MORTAL SIN'.
'by such a serious' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for mortal sin that I've seen before include "heinous act" , "It causes the death of the soul - not venial" .)