Tool writer observed (7)
I believe the answer is:
hacksaw
'tool' is the definition.
(hacksaw is a kind of tool)
'writer observed' is the wordplay.
'writer' becomes 'hack' (hack can mean a journalist).
'observed' becomes 'saw' (seen).
'hack'+'saw'='HACKSAW'
(Other definitions for hacksaw that I've seen before include "Blade in a frame under tension" , "Metal cutter" , "Toothed tool for cutting metal" , "means of cutting lead article?" , "cutting edge" .)