Discover doctor breaking law (6)
I believe the answer is:
rumble
'discover' is the definition.
(to rumble someone is to discover their secret)
'doctor breaking law' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine).
'breaking' is an insertion indicator.
'law' becomes 'rule' (rule is a kind of law).
'mb' put into '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" .)