Treat old carbon in newspapers (7)
I believe the answer is:
process
'treat' is the definition.
(I know that treat can be written as process)
'old carbon in newspapers' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'carbon' becomes 'C' (chemical symbol).
'in' means one lot of letters goes inside another.
'newspapers' becomes 'press' (the press is the newspaper industry).
'o'+'c'='oc'
'oc' going into 'press' is 'PROCESS'.
(Other definitions for process that I've seen before include "Series of actions to achieve a result" , "Method of operation" , "Series of actions with a single purpose" , "Deal with, action" , "Natural series of changes" .)