Do something in law (3)
I believe the answer is:
act
I believe this is a double definition.
'do something' is the first definition.
(I've seen this before)
'law' is the second definition.
(thesaurus)
'in' acts as a link.
(Other definitions for act that I've seen before include "Take steps" , "Feign" , "Take part in a play" , "Start of a drama" , "Feat" .)