A lot of papers include Latin in their field of study (5)
I believe the answer is:
realm
'field of study' is the definition.
(I've seen this before)
'a lot of papers include latin in' is the wordplay.
'lot of papers' becomes 'rem' (I can't explain this - if you can you should give a lot more credence to this answer).
'include' means one lot of letters goes inside another.
'latin' becomes 'l'.
'in' is an insertion indicator.
'rem' enclosing 'l' is 'relm'.
'a' inserted into 'relm' is 'REALM'.
'their' acts as a link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for realm that I've seen before include "Kingdom; field of interest" , "region" , "field of study" , "World" , "department" .)