Class invaded by large rook (6)
I believe the answer is:
castle
'rook' is the definition.
('castle' can be a synonym of 'rook')
'class invaded by large' is the wordplay.
'class' becomes 'caste' (caste is a kind of class).
'invaded by' means one lot of letters goes inside another.
'large' becomes 'l'.
'caste' going around 'l' is 'CASTLE'.
(Other definitions for castle that I've seen before include "fortification" , "Another word for chess rook" , "Large fortified building" , "Left's Social Services secretary" , "Large building; rook" .)