Black donkey - one with a low voice (4)
I believe the answer is:
bass
'low voice' is the definition.
(I've seen this before)
'black donkey' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'donkey' becomes 'ass' (ass is a term for a donkey).
'b'+'ass'='BASS'
'one with a' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for bass that I've seen before include "Low-pitched; a fish" , "Fish; deep voice" , "part of score" , "Lowest adult male singing voice" , "Large fish - singer" .)