Shocked initially and greatly horrified at such treatment (6)
I believe the answer is:
aghast
'shocked' is the definition.
(I know that shocked can be written as aghast)
'initially and greatly horrified at such treatment' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'and greatly horrified at such treatment' is 'AGHAST'.
(Other definitions for aghast that I've seen before include "in awe" , "Horror-struck" , "Stupefied with horrer" , "Stunned" , "What's carried in bag has teacher" .)