Language rules in cryptogram marked (7)
I believe the answer is:
grammar
'language rules' is the definition.
(I've seen this before)
'in cryptogram marked' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GRAMMAR' is hidden within 'cryptogram marked'.
(Other definitions for grammar that I've seen before include "Rules for words formation and structures" , "Science of language" , "Study of word and sentences" , "type of school" , "Correct usage of words" .)