Twisting force (6)
I believe the answer is:
torque
'twisting force' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for torque that I've seen before include "Source of spin" , "Force that produces rotation" , "Metal necklace; also a measure of force" , "Force causing rotation in mechanism" , "measure of twist" .)