Special knowledge of a sound law (4)
I believe the answer is:
lore
'special knowledge' is the definition.
'lore' can be an answer for 'knowledge' (lore is a kind of knowledge). I am not certain of the 'special' bit.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for lore that I've seen before include "Old learning" , "Role for learning" , "Wisdom" , "Ancient learning" , "Role for body of traditions" .)