Impulse to stop boy becoming doctor (7)
I believe the answer is:
surgeon
'doctor' is the definition.
(I know that doctor can be written as surgeon)
'impulse to stop boy' is the wordplay.
'impulse' becomes 'urge' (synonyms).
'to stop' indicates putting letters inside.
'boy' becomes 'son' (synonyms).
'urge' inserted within 'son' is 'SURGEON'.
'becoming' acts as a link.
(Other definitions for surgeon that I've seen before include "Naval medic" , "Theatre worker" , "One who operates" , "Performer of operations" , "Doctor with cutting tools" .)