Classification charge possible? I'm dismayed (8)
I believe the answer is:
taxonomy
'classification' is the definition.
(I know that taxonomy is a type of classification)
'charge possible? i'm dismayed' is the wordplay.
'charge' becomes 'tax' (tax is a kind of charge).
'possible?' becomes 'on' ('not on' can mean unacceptable or not possible).
'i'm dismayed' becomes 'O my' (expression of dismay).
'tax'+'on'+'omy'='TAXONOMY'
(Other definitions for taxonomy that I've seen before include "The scientific process of classifying living things" , "Science of classification" , "System of classification" .)