Clue: Ring deters wanton magnetism (7)
Answer: OERSTED
Explanation: Definition: magnetism
O + *(deters) 
An oersted is the unit of the auxiliary magnetic field H in the CGS system of units. Apart from it being named after a Danish phycist, that's all I know about it, so don't know if it can be clued as "magnetism" or whether it should be "a unit of magnetism", which would not work in the surface of the clue.