A black bird that is a green one to soldiers (6)
I believe the answer is:
rookie
'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" .)