Model prisoner first of those getting out of nick (6)
I believe the answer is:
trusty
'model prisoner' is the definition.
(informal term for a reliable prisoner)
'first of those getting out of nick' is the wordplay.
'first of' suggests taking the first letters.
'getting' says to put letters next to each other.
'out of nick' becomes 'rusty' (both can mean out of practice).
The initial letter of 'those' is 't'.
't'+'rusty'='TRUSTY'
(Other definitions for trusty that I've seen before include "Reliable (prisoner)" , "Trainee officer" , "Staunch" , "Dependable" , "Reliable, honest and firm" .)