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