Rules lines on carbon (4)
I believe the answer is:
code
'rules' is the definition.
'lines on carbon' is the wordplay.
'lines' becomes 'ode' (I've seen this before**).
'on' says to put letters next to each other.
'carbon' becomes 'c' (C is the chemical symbol for carbon).
'ode' after 'c' is 'CODE'.
(Other definitions for code that I've seen before include "Cryptogram or set of standards" , "Secret symbols" , "ethical principles?" , "Program instructions" , "regulations" .)