First in history, sitting in class, modest (6)
I believe the answer is:
chaste
'modest' is the definition.
(thesaurus)
'first in history sitting in class' is the wordplay.
'first in' says to take the initial letters.
'sitting in' is an insertion indicator.
'class' becomes 'caste' (caste is a kind of class).
The initial letter of 'history' is 'h'.
'h' put inside 'caste' is 'CHASTE'.
(Other definitions for chaste that I've seen before include "Modest, pure" , "sober" , "Untarnished" , "He-cats could be virtuous, strangely" , "Sachet (anag)" .)