The estrogen receptor requires binding of an estrogen or drug at its binding site and also the binding of any of several cofactors at different sites. Without the binding of the cofactor, the estrogen receptor is inactive.
Different tissues use different cofactors.
Clomid is an effective antagonist in the hypothalamus and in breast tissue. It is an effective agonist in bone tissue, and for improving blood cholesterol. Clomid also has the property of reducing the adverse effect of exercise-induced damage of muscle tissue. This is very significant for endurance athletes but is not very significant, if at all significant, with reasonable weight training. Clomid does not perceptibly affect gains of the weight trainer either favorably or adversely in my experience.