Frisk doctor in prison (6)
I believe the answer is:
gambol
'frisk' is the definition.
('gambol' can be a synonym of 'frisk')
'doctor in prison' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'in' indicates putting letters inside.
'prison' becomes 'gaol' (I've seen this before).
'mb' put into 'gaol' is 'GAMBOL'.
(Other definitions for gambol that I've seen before include "Skip" , "Frisk about and bet, by the sound of it" , "jump for joy" , "Playfully run or jump" , "Frolic, caper" .)