Tool spotted by poor writer (7)
I believe the answer is:
hacksaw
'tool' is the definition.
(hacksaw is a kind of tool)
'spotted by poor writer' is the wordplay.
'spotted' becomes 'saw' (spot can mean to see).
'by' is a charade indicator (letters next to each other).
'poor writer' becomes 'hack' (I've seen this in another clue).
'saw' put after 'hack' is 'HACKSAW'.
(Other definitions for hacksaw that I've seen before include "metalworker may use?" , "Metal cutter" , "something to cut copper?" , "Blade in a frame under tension" , "One cuts" .)