Unusually nasty adverse mark for use of language (6)
I believe the answer is:
syntax
'language' is the definition.
(syntax language is a kind of language)
'unusually nasty adverse mark for use' is the wordplay.
I cannot quite see how this works, but
an anagram of 'nasty' is 'synta' which is located in the answer.
A single letter 'x' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for syntax that I've seen before include "Grammatical rules" , "The rules governing the arrangement of words in sentences etc" , "Verbal order" , "Linguistic study" , "Arrangement of words and phrases in a sentence" .)