A black bird that is a green one to soldiers (6)

I believe the answer is:

rookie

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'a black bird that is a green one to soldiers' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

'a black bird that is' is the wordplay.
'a black bird' becomes 'rook' (I've seen this before).
'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 "untested type" , "Slang for a new recruit" , "untrained soldier" , "Army or police recruit" , "New recruit as to army" .)

I've seen this clue in The Globe and Mail.

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