Chess piece left in class (6)
I believe the answer is:
castle
'chess piece' is the definition.
(castle is a kind of chess piece)
'left in class' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'class' becomes 'caste' (caste is a kind of class).
'l' going within 'caste' is 'CASTLE'.
(Other definitions for castle that I've seen before include "Move two pieces" , "Chess piece and grand house" , "Large building; rook" , "Chateau" , "Chess piece and fortified building" .)