Bold prisoner able to conceal hideaway (9)
I believe the answer is:
confident
'bold' is the definition.
('confident' can be similar in meaning to 'bold')
'prisoner able to conceal hideaway' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'able' becomes 'fit' ('fit' can be similar in meaning to 'able').
'to conceal' indicates putting letters inside.
'hideaway' becomes 'den' ('den' can be a synonym of 'hideaway').
'fit' enclosing 'den' is 'fident'.
'con'+'fident'='CONFIDENT'
(Other definitions for confident that I've seen before include "Feeling certain" , "Trusting firmly" , "5 [CERTAIN]" , "Assured, positive" , "Bold" .)