Grammatical rules (6)
I believe the answer is:
syntax
'grammatical rules' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for syntax that I've seen before include "branch of linguistics" , "Grammatical rules" , "The rules governing the arranging of words" , "Way in which words are arranged to form sentences" , "The rules governing words arrangements" .)