Current judge retires after initially libelling prisoner (5)
I believe the answer is:
lifer
'prisoner' is the definition.
(lifer is a kind of prisoner)
'current judge retires after initially libelling' is the wordplay.
'current' becomes 'i' (symbol for electric current).
'judge' becomes 'ref'.
'retires' shows that the letters should be reversed in order.
'after' says to put letters next to each other.
'initially' suggests taking the first letters.
The initial letter of 'libelling' is 'l'.
'ref' reversed gives 'fer'.
'i'+'fer'='ifer'
'ifer' after 'l' is 'LIFER'.
(Other definitions for lifer that I've seen before include "One behind bars for ever" , "Rifle (anag.)" , "One with very long sentence" , "One with maximum sentence" , "One given lots of porridge" .)