Don't open post: deliver note (7)
I believe the answer is:
observe
'note' is the definition.
('observe' can be a synonym of 'note')
'don't open post deliver' is the wordplay.
I cannot really understand how this works, but
'deliver' could be 'serve' (serving is a kind of delivering) and 'serve' is found in the answer.
The remaining letters 'ob' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for observe that I've seen before include "See - say" , "Watch closely" , "Notice, watch" , "Keep; notice" , "Mark" .)