Confound prisoner with safety device (7)
I believe the answer is:
confuse
'confound' is the definition.
(I know that confound can be written as confuse)
'prisoner with safety device' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'with' is a charade indicator (letters next to each other).
'safety device' becomes 'fuse' (electronic safety device).
'con'+'fuse'='CONFUSE'
(Other definitions for confuse that I've seen before include "Muddle, mix up" , "Disconcert, befuddle" , "Perplex" , "Bewilder" , "Puzzle" .)