Crossword Genius

Piece that is for a beginner (6)

Ross

I believe the answer is:

rookie

'for a beginner' is the definition.
(rookie is a kind of beginner)

'piece that is' is the wordplay.
'piece' becomes 'rook' (rook is a kind of chess piece).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'rook'+'ie'='ROOKIE'

(Other definitions for rookie that I've seen before include "Novice" , "untested type" , "Callow recruit" , "Beginner as in the army" , "Raw recruit" .)

I've seen this clue in The Telegraph and The Times.
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