Dull writer uses proverb as a tool (7)
I believe the answer is:
hacksaw
'a tool' is the definition.
(hacksaw is a kind of tool)
'dull writer uses proverb' is the wordplay.
I cannot quite see how this works, but
'writer' could be 'hack' (hack can mean a journalist) and 'hack' is located in the answer.
'proverb' could be 'saw' (synonyms) and 'saw' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'as' is the link.
Can you help me to learn more?
(Other definitions for hacksaw that I've seen before include "Metal cutter" , "Blade in a frame under tension" , "Toothed tool for cutting metal" , "7 for metal" , "Metal-cutting tool" .)