Header seen across such post would be 'Top Teacher' (4)
I believe the answer is:
head
This clue is a sextuple definition!!!
'header' is the first definition.
(I know that header can be written as head)
'seen across such post' is the second definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'would' is the third definition.
The definition and answer can be both to do with fundamental existence as well as being verbs in their base form.
Perhaps you can see a link between them that I can't see?
'be' is the fourth definition.
(heading is a kind of being)
'top' is the fifth definition.
(synonyms)
'teacher' is the sixth definition.
(I know that head is a type of teacher)
Can you help me to learn more?
(Other definitions for head that I've seen before include "Beer foam; front" , "Person in charge" , "Edible part of cabbage" , "Leader; water pressure" , "River or stream source" .)