Pens ran out; that’s a wrench (7)
I believe the answer is:
spanner
'that's a wrench' is the definition.
('spanner' can be a synonym of 'wrench')
'pens ran out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'pens'+'ran'='pensran'
'pensran' is an anagram of 'SPANNER'.
(Other definitions for spanner that I've seen before include "that might cause difficulty in the works" , "Hand tool for nuts" , "Nut-tightening tool" , "Tool for gripping nut" , "Wrench for nuts" .)