Tool used to be written up by journalist (7)
I believe the answer is:
hacksaw
'tool' is the definition.
(hacksaw is a kind of tool)
'used to be written up by journalist' is the wordplay.
'used to be' becomes 'was' (eg 'he was...' means 'he used to be...').
'written up' says the letters should be written backwards.
'by' is a charade indicator (letters next to each other).
'journalist' becomes 'hack' (hack can informally mean a journalist).
'was' in reverse letter order is 'saw'.
'saw' after 'hack' is 'HACKSAW'.
(Other definitions for hacksaw that I've seen before include "Metal-cutting tool" , "metalworker may use?" , "One cuts" , "cutting edge" , "7 for metal" .)