Penetrate after having sacrificed rook or knight? (5)
I believe the answer is:
piece
'knight?' is the definition.
(knight is a kind of piece)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for piece that I've seen before include "one of eight once ready for the Spanish?" , "bishop on board, say" , "that may follow show after work party?" , "Unpleasant character" , "American gun" .)