Boggle the mind, as prisoner discovered (8)
I believe the answer is:
confound
'boggle the mind' is the definition.
'confound' can be an answer for 'boggle' (thesaurus). I'm not sure about the 'the mind' bit.
'prisoner discovered' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'discovered' becomes 'found' (to discover is to find).
'con'+'found'='CONFOUND'
'as' is the link.
Can you help me to learn more?
(Other definitions for confound that I've seen before include "Mix up" , "Puzzle" , "Bewilder" , "Defeat (a plan or hope)" , "Nonplus" .)