Very good workers do - and very bad ones (7,6)

'very good workers' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'do and very bad ones' is the wordplay.
I cannot quite see how this works, but
'do' could be 'serve' (serving is a kind of doing) and 'serve' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?