It's inconceivable academic keeps large quantity of paper (5,2)
I believe the answer is:
dream on
'it's inconceivable' is the definition.
(said in response to something unfeasible)
'academic keeps large quantity of paper' is the wordplay.
'academic' becomes 'don' (type of academic - especially at Oxbridge).
'keeps' means one lot of letters goes inside another.
'large quantity of paper' becomes 'ream' (a ream is a large amount of paper).
'don' going around 'ream' is 'DREAM ON'.
(Other definitions for dream on that I've seen before include "Come off it!" , "That's unlikely! (colloquial)" .)