Serious transgression? It's normal, possibly (6,3)
I believe the answer is:
mortal sin
'serious transgression?' is the definition.
(a serious sin)
'it's normal possibly' is the wordplay.
'possibly' is an anagram indicator.
'its'+'normal'='itsnormal'
'itsnormal' with letters rearranged gives 'MORTAL SIN'.
(Other definitions for mortal sin that I've seen before include "It causes the death of the soul - not venial" , "heinous act" .)