Terrible starts for anyone who files unread letters (5)
I believe the answer is:
awful
'terrible' is the definition.
(I know that terrible can be written as awful)
'starts for anyone who files unread letters' is the wordplay.
'starts for' says to take the initial letters (I've seen 'start for' mean this).
The first letters of 'anyone who files unread letters' is 'AWFUL'.
(Other definitions for awful that I've seen before include "Very poor" , "pants" , "Really terrible, deplorable" , "Exceptionally bad" , "Very embarrassing" .)