object follow mouse in radius

1) Get mouse position into world space:

 Transform player;
 Vector3 wPos = Input.mousePosition;
 wPos.z = player.position.z-Camera.main.transform.position.z;
 wPos = Camera.main.ScreenToWorldPoint(wPos);

2) Get direction from player to worldMousePosition

 Vector3 direction = wPos - player.position;

3) Clamp magnitude to certain radius from player

 float radius = 5;
 direction = Vector3.ClampMagnitude(direction. radius);

4) set object position

 Transform someObject;
 someObject.position = player.position+direction;