The "C" in UPC (4)
I believe the answer is:
code
'the c in upc' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for code that I've seen before include "Program instruction" , "Set of signals for sending messages" , "See 12" , "Method of encryption or set of rules" , "System of rules" .)