Concerning a mass quantity of paper (4)
I believe the answer is:
ream
'quantity of paper' is the definition.
(I've seen this before)
'concerning a mass' is the wordplay.
'concerning' becomes 're'.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
're'+'a'+'m'='REAM'
(Other definitions for ream that I've seen before include "Lots of paper" , "Many sheets - enlarge hole" , "Widen (hole); quantity of paper" , "516 sheets of paper" , "Widen out (bore)" .)