Inferior alternative to the Post Office (4)
I believe the answer is:
poor
'inferior' is the definition.
('poor' can be similar in meaning to 'inferior')
'alternative to the post office' is the wordplay.
'alternative' becomes 'or'.
'to' means one lot of letters go next to another (I've seen this in other clues).
'the post office' becomes 'po'.
'or' after 'po' is 'POOR'.
(Other definitions for poor that I've seen before include "Low-quality" , "mean" , "Penurious" , "Needy; meagre" , "hopeless" .)