Defence post (4)
I believe the answer is:
This clue could be a double definition.
'defence' is the first definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'post' is the second definition.
(I know that post can be written as mail)
Can you help me to learn more?
(Other definitions for mail that I've seen before include "Paper" , "Form of armour; post" , "Post or body armour" , "American post from Lima" , "some self-defence material" .)