Open prison served the purpose (6)
I believe the answer is:
candid
'open' is the definition.
(I know that open can be written as candid)
'prison served the purpose' is the wordplay.
'prison' becomes 'can' (the can is American slang for prison).
'served the purpose' becomes 'did' (I've seen this in another clue).
'can'+'did'='CANDID'
(Other definitions for candid that I've seen before include "Frank, honest" , "Truthful, frank" , "Honest and open about what one thinks" , "Free and frank" , "Frank - unbiased" .)