Erroneously file a letter for a prisoner (5)
I believe the answer is:
lifer
'a prisoner' is the definition.
(lifer is a kind of prisoner)
'erroneously file a letter' is the wordplay.
'erroneously' indicates anagramming the letters.
'a letter' becomes 'r' ().
'file' anagrammed gives 'life'.
'life'+'r'='LIFER'
'for' acts as a link.
(Other definitions for lifer that I've seen before include "One given lots of porridge" , "Con" , "One behind bars for ever" , "One with very long sentence" , "Person serving long sentence" .)