Attorney's expertise (3)
I believe the answer is:
law
'attorney's expertise' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for law that I've seen before include "Commandment" , "Set of rules laid down by authority" , "Body of rules of conduct for a society" , "Rule imposed by authority" , "Collection of rules imposed by authority" .)