Erroneously file a letter for a prisoner (5)
I believe the answer is:
lifer
'prisoner' is the definition.
(prisoner serving a life sentence)
'erroneously file a letter' is the wordplay.
'erroneously' indicates an anagram.
'a letter' becomes 'r' ().
'file' is an anagram of 'life'.
'life'+'r'='LIFER'
'for a' is the 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" .)