Writers can be terrible nags (5)
I believe the answer is:
hacks
I believe this clue is a double definition.
'writers' is the first definition.
(hack is a kind of writer)
'be terrible nags' is the second definition.
(hack can mean a worn out horse)
'can' acts as a link.
(Other definitions for hacks that I've seen before include "Chops at like routine writers" , "Mediocre writers" , "Coughs" , "Press" , "Uninspired writers" .)