Reliable prisoner loses head, being out of practice (5)
I believe the answer is:
rusty
'out of practice' is the definition.
(I've seen this before)
'reliable prisoner loses head' is the wordplay.
'reliable prisoner' becomes 'trusty' (I've seen this before).
'loses head' means to remove the first letter.
'trusty' with its initial letter taken off is 'RUSTY'.
'being' acts as a link.
(Other definitions for rusty that I've seen before include "Having a reddish-orange oxide coating" , "a bit faded" , "(Of iron) corroded" , "Behind the times" , "(Of iron) affected by damp" .)