Discover the truth about doctor breaking law (6)
I believe the answer is:
rumble
'discover the truth about' is the definition.
(I've seen this before)
'doctor breaking law' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
'breaking' means one lot of letters goes inside another.
'law' becomes 'rule' (rule is a kind of law).
'mb' going into 'rule' is 'RUMBLE'.
(Other definitions for rumble that I've seen before include "Find out the truth" , "Discover (illicit activity); continuous deep noise" , "street-fight between gangs" , "Such a strip" , "Tummy sound - discover (an illicit activity)" .)