Satisfy prisoner failing to pen note (8)
I believe the answer is:
convince
'satisfy' is the definition.
(I've seen this before)
'prisoner failing to pen note' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'failing' becomes 'vice' (vice can mean a moral failing).
'to pen' indicates putting letters inside.
'note' becomes 'n' (abbreviation).
'vice' placed around 'n' is 'vince'.
'con'+'vince'='CONVINCE'
(Other definitions for convince that I've seen before include "Persuade successfully" , "Persuade someone of the truth of something" , "Make believe" , "Satisfy by evidence" , "Win over to your point of view" .)