How fit could a prisoner be? (2,4,4)
I believe the answer is:
in good nick
'how fit' is the definition.
(something in good nick is in good condition)
'a prisoner be?' is the wordplay.
I cannot really see how this works, but
'be?' could be 'go' (going is a kind of being) and 'go' is found in the answer.
This explanation may well be incorrect...
'could' is the link.
(Other definitions for in good nick that I've seen before include "fine" , "punished by benign penal system?" , "Where honest cops are based" , "Where one enjoys porridge" , "doing time here?" .)