Piece that is new to the game (6)
I believe the answer is:
rookie
'piece that is new to the game' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'piece that is' is the wordplay.
'piece' becomes 'rook' (rook is a kind of chess piece).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'rook'+'ie'='ROOKIE'
Can you help me to learn more?
(Other definitions for rookie that I've seen before include "Novice" , "Raw recruit" , "new team member" , "untrained soldier" , "Slang for a new recruit" .)