Piece of data, a single line added to one paper (10)
I believe the answer is:
stationery
'paper' is the definition.
(stationery is a kind of paper)
'piece of data a single line added to one' is the wordplay.
'piece of data' becomes 'stat' (short for statistic).
'a single' becomes 'I' (one).
'line' becomes 'ry' (abbreviation for railway).
'added to' is a charade indicator (letters next to each other).
'ry' put after 'one' is 'onery'.
'stat'+'i'+'onery'='STATIONERY'
(Other definitions for stationery that I've seen before include "The papers, etc" , "Materials needed for writing" , "Various items in paper" , "Files etc" , "Pens, pencils, paper etc" .)