Piece that is for one who's just started (6)

I believe the answer is:

rookie

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'piece that is for one who's just started' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

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

Can you help me to learn more?

(Other definitions for rookie that I've seen before include "Callow recruit" , "Slang for a new recruit" , "new team member" , "Beginner as in the army" , "Army or police recruit" .)

I've seen this clue in The Telegraph.

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