Rules regarding language ultimately applying to crash damage (7)
I believe the answer is:
grammar
'rules regarding language' is the definition.
'grammar' can be an answer for 'rules' (I've seen this before). I am not certain of the 'regarding language' bit.
'ultimately applying to crash damage' is the wordplay.
'ultimately' says to take the final letters.
'to crash' becomes 'ram' ('ram' can be a synonym of 'crash').
'damage' becomes 'mar' (to mar is to spoil or damage).
The last letter of 'applying' is 'g'.
'g'+'ram'+'mar'='GRAMMAR'
Can you help me to learn more?
(Other definitions for grammar that I've seen before include "Branch of linguistics that deals with syntax" , "Science of language" , "Correct word use" , "Study of basic linguistic units" , "Branch of language study" .)