Academic soldiers are upset over low mark (6)
I believe the answer is:
reader
'academic' is the definition.
(I've seen this before)
'soldiers are upset over low mark' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'are upset over' says the letters should be written in reverse (knocked over).
'low mark' becomes 'reda' (I can't explain this - if you can you should give a lot more credence to this answer).
'reda' reversed gives 'ader'.
're'+'ader'='READER'
Can you help me to learn more?
(Other definitions for reader that I've seen before include "member of the university" , "University lecturer, ranking below a professor" , "University academic" , "Lecturer; book user" , "One who studies - or his book" .)