Rotational force; necklace (6)
I believe the answer is:
torque
I believe this clue is a double definition.
'rotational force' is the first definition.
(I've seen this before)
'necklace' is the second definition.
(I know that torque is a type of necklace)
(Other definitions for torque that I've seen before include "Force that produces rotation" , "Force tending to turn" , "Rotational force; roquet (anag.)" , "Quoter (anag.)" , "measure of twist" .)