When you say 'constrain' are you implying that if you move what I assume would be the Datum CSYS that the circle would move as well? If so, then you will need to create a Sketch on the X-Y plane of the Datum CSYS and then create your circle, constraining it's center to be 'Point On Curve' selecting the Datum Y-Axis as the 'curve'.
Now if all you want to do is simply create a circle whose center just HAPPENS to lie on the Y-axis, just go to...
Insert -> Curve -> Lines and Arcs -> Circle Center-Radius...
...and when the on-screen entry is displayed showing the XC,YC,ZC coordinates as you move the cursor around the screen, just enter a '0' for the value of 'XC', press MB1, then either drag or enter a radius value and then press MB1 a second time.
John R. Baker, P.E.
Product 'Evangelist'
Product Design Solutions
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
