Advocate stops son becoming medical practitioner (7)
I believe the answer is:
surgeon
'medical practitioner' is the definition.
(surgeon is a kind of medical practitioner)
'advocate stops son' is the wordplay.
'advocate' becomes 'urge' (synonyms).
'stops' means one lot of letters goes inside another (stopping can mean blocking a gap).
'urge' put inside 'son' is 'SURGEON'.
'becoming' is the link.
(Other definitions for surgeon that I've seen before include "one used to working in the theatre" , "One who operates" , "Physician" , "One operating in theatre" , "Operating medic" .)