Discover doctor breaking law (6)
I believe the answer is:
rumble
'discover' is the definition.
(I've seen this before)
'doctor breaking law' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
'breaking' is an insertion indicator.
'law' becomes 'rule' (rule is a kind of law).
'mb' going inside 'rule' is 'RUMBLE'.
(Other definitions for rumble that I've seen before include "Such a strip" , "Tummy sound - discover (an illicit activity)" , "Make a continuous resonant sound" , "be wise to" , "Harlem gangs might" .)