Chair one found in class (4)
I believe the answer is:
seat
'chair' is the definition.
(thesaurus)
'one found in class' is the wordplay.
'one' becomes 'a'.
'found in' is an insertion indicator.
'class' becomes 'set' (I've seen this before).
'a' put inside 'set' is 'SEAT'.
(Other definitions for seat that I've seen before include "MP's place in parliament" , "Sitting place" , "Spanish car" , "A manner of sitting on horse" , "Parliamentary constituency" .)