Black bird dog (6)

I believe the answer is:

beagle

'dog' is the definition.
(I know that beagle is a dog breed)

'black bird' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'bird' becomes 'eagle' (eagle is a type of bird).
'b'+'eagle'='BEAGLE'

(Other definitions for beagle that I've seen before include "Hunting hound" , "Short-haired dog" , "5 [MANS BEST FRIEND]" , "Small short-legged hound" , "Type of hound" .)

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

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