Rotational force (6)
I believe the answer is:
torque
'rotational force' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for torque that I've seen before include "Source of spin" , "Quoter (anag.)" , "Rotational force; roquet (anag.)" , "Turning force" , "Rotating forces" .)