Inferior piece of work by the post office (4)
I believe the answer is:
poor
'inferior' is the definition.
(similar in meaning)
'piece of work by the post office' is the wordplay.
'piece of work' becomes 'or' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' means one lot of letters go next to another.
'the post office' becomes 'po'.
'or' after 'po' is 'POOR'.
(Other definitions for poor that I've seen before include "Hard-up" , "They used to have a law" , "Not good, wealthy" , "Lacking money and resources" , "Shoddy" .)