Restrained introduction to history in class (6)
I believe the answer is:
chaste
'restrained' is the definition.
(chaste can mean restrained or simple)
'introduction to history in class' is the wordplay.
'introduction to' indicates taking the first letters.
'in' indicates putting letters inside.
'class' becomes 'caste' (type of social class).
The first letter of 'history' is 'h'.
'h' going into 'caste' is 'CHASTE'.
(Other definitions for chaste that I've seen before include "Undefiled" , "Modest, pure" , "He-cats could be virtuous, strangely" , "Morally pure" , "sober" .)