Nut-turning tool (7)
I believe the answer is:
spanner
'nut-turning tool' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for spanner that I've seen before include "Tool for gripping and turning" , "Bad thing to have in the works!" , "Nut-tightening tool" , "Nut turner" , "Tool for tightening or loosening" .)