The key to get out of prison? (6)
I believe the answer is:
escape
This clue is a double definition.
'the key' is the first definition.
(key on a keyboard)
'get out of prison?' is the second definition.
(a prisoner might escape)
'to' acts as a link.
(Other definitions for escape that I've seen before include "See cap (anag.)" , "Decamp" , "Leak; avoid" , "Break free from confinement" , "Gain freedom" .)