Lark putting doctor into prison (6)
I believe the answer is:
gambol
'lark' is the definition.
('gambol' can be a synonym of 'lark')
'doctor into prison' is the wordplay.
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'into' means one lot of letters goes inside another.
'prison' becomes 'gaol' (I've seen this before).
'mb' going inside 'gaol' is 'GAMBOL'.
'putting' is the link.
(Other definitions for gambol that I've seen before include "Spring" , "jump for joy" , "Frisk, frolic" , "Frisk about and bet, by the sound of it" , "Prance; sounds like bet" .)