Frisk doctor entering prison (6)
I believe the answer is:
gambol
'frisk' is the definition.
(synonyms)
'doctor entering prison' is the wordplay.
'doctor' becomes 'MB' (Bachelor of Medicine).
'entering' indicates putting letters inside.
'prison' becomes 'gaol' (I've seen this before).
'mb' placed within 'gaol' is 'GAMBOL'.
(Other definitions for gambol that I've seen before include "Playfully run or jump" , "Wager a frisk, by the sound of it" , "Frisk about and bet, by the sound of it" , "Frolic, caper" , "Prance; sounds like bet" .)