Out of practice, well-behaved prisoner loses head (5)
I believe the answer is:
rusty
'out of practice' is the definition.
(synonyms)
'well-behaved prisoner loses head' is the wordplay.
'well-behaved prisoner' becomes 'trusty' (prisoner rewarded with special privileges).
'loses head' means to remove the first letter.
'trusty' with its first letter taken away is 'RUSTY'.
(Other definitions for rusty that I've seen before include "Stiff with age" , "a bit faded" , "Impaired through lack of use" , "Having a reddish-orange oxide coating" , "(Of iron) corroded" .)