Criminal law - that's boring (3)
I believe the answer is:
awl
'boring' is the definition.
(I've seen this before)
'criminal law' is the wordplay.
'criminal' indicates anagramming the letters.
'law' is an anagram of 'AWL'.
'that's' acts as a link.
(Other definitions for awl that I've seen before include "Sharp point" , "Law about a drilling tool" , "What could be boring" , "it may be piercing" , "One piercing" .)