Put off protecting prison vessel (8)
I believe the answer is:
decanter
'vessel' is the definition.
(decanter is a kind of vessel)
'put off protecting prison' is the wordplay.
'put off' becomes 'deter' (I've seen this before).
'protecting' means one lot of letters goes inside another.
'prison' becomes 'can' (the can is American slang for prison).
'deter' going around 'can' is 'DECANTER'.
(Other definitions for decanter that I've seen before include "Vessel with stopper for serving wine" , "quantity of wine?" , "Server" , "bottle needed" , "Wine/spirits container" .)