Paper in mass cut back (4)
I believe the answer is:
exam
'paper' is the definition.
(I've seen this before)
'mass cut back' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'cut' becomes 'axe' (both can mean to cancel or remove).
'back' is a reversal indicator.
'm'+'axe'='maxe'
'maxe' reversed gives 'EXAM'.
'in' acts as a link.
(Other definitions for exam that I've seen before include "Oral maybe" , "Set of questions evaluating skill or knowledge, in short" , "School test (colloq.)" , "'Test, in short (4)'" , "Written test" .)