Hard to be expelled from virtuous class (5)
I believe the answer is:
caste
'class' is the definition.
(type of social class)
'hard to be expelled from virtuous' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to be expelled from' indicates named letters should be taken away.
'virtuous' becomes 'chaste' (similar in meaning).
'chaste' with 'h' taken away is 'CASTE'.
(Other definitions for caste that I've seen before include "Religious or social class" , "Social division in India" , "Group within society" , "Social order" , "Social class or rank" .)