Habitual response about acceptable in public school language once (8)
I believe the answer is:
teutonic
'language once' is the definition.
(historical European language)
'habitual response about acceptable in public school' is the wordplay.
'habitual response' becomes 'tic' (an involuntary physical movement).
'about' is an insertion indicator.
'acceptable' becomes 'u' (as in U and non-U speech).
'in' means one lot of letters goes inside another.
'public school' becomes 'eton' (Eton College).
'u' going inside 'eton' is 'euton'.
'tic' going around 'euton' is 'TEUTONIC'.
(Other definitions for teutonic that I've seen before include "German (e.g. thoroughness)" , "Of the Germanic peoples" , "Like some ancient Germans" , "Relating to the Germanic language" , "Characteristic of Germans" .)