Manipulate[
Module[{sol = NDSolve[{y''[x]+Sin[y[x]]==0,y[0]==p[[1]],y'[0]==p[[2]]}, y, {x,0,T}]},
ParametricPlot[Evaluate[{y[x], y'[x]} /. sol], {x,0,T}, PlotRange->{{-4,4},{-3,3}}]],
{{p, {2,1}}, Locator}, {{T,10},0,100}]