Field marshal's back in the papers (5)
I believe the answer is:
realm
'field' is the definition.
(I know that realm is a type of field)
'marshal's back in the papers' is the wordplay.
'back' indicates one should take the final letters.
'in the' is an insertion indicator.
'papers' becomes 'ream' (a ream is a large amount of paper).
The last letter of 'marshal' is 'l'.
'l' inserted into 'ream' is 'REALM'.
(Other definitions for realm that I've seen before include "Area of knowledge under discussion" , "Royal domain" , "Kingdom, domain" , "the UK, perhaps" , "department" .)