Data page fed into processing unit (5)
I believe the answer is:
input
'data' is the definition.
(data put into a system)
'page fed into processing unit' is the wordplay.
'page' becomes 'p'.
'fed into' indicates putting letters inside.
'processing' indicates anagramming the letters.
'unit' anagrammed gives 'inut'.
'p' put inside 'inut' is 'INPUT'.
(Other definitions for input that I've seen before include "Simple, without artifice" , "Signal feeding a circuit" , "data-feed" , "Provision of data" , "Signal entering a circuit" .)