Quantity of paper covering large field of study (5)
I believe the answer is:
realm
'field of study' is the definition.
(I've seen this before)
'quantity of paper covering large' is the wordplay.
'quantity of paper' becomes 'ream' (a ream is a large amount of paper).
'covering' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'ream' enclosing 'l' is 'REALM'.
(Other definitions for realm that I've seen before include "Kingdom, domain" , "Knowledge base" , "Land under a king or queen" , "the UK, perhaps" , "Province" .)