Treatment applied to host, as in shock (8)
I believe the answer is:
astonish
'shock' is the definition.
(I've seen this before)
'treatment applied to host as in' is the wordplay.
'treatment applied to' is an anagram indicator (the letters must be processed or treated).
'host'+'as'+'in'='hostasin'
'hostasin' with letters rearranged gives 'ASTONISH'.
(Other definitions for astonish that I've seen before include "Take by surprise" , "Shock with surprise" , "Greatly impress" , "Startle" , "Amaze, astound" .)