The ornamental design for a closure for a containerneck, as shown and described.
FIG. 1 is a top-front-left perspective view of a closure for a container neck bearing the design;
FIG. 2 is an elevational, front view of the closure of FIG. 1;
FIG. 3 is an elevational, rear view of the closure of FIG. 1;
FIG. 4 is an elevational, right view of the closure of FIG. 1; and,
FIG. 5 is a top, plan view of the closure of FIG. 1.
An elevational, left view of the closure is a mirror image of the elevational, right view of FIG. 4. The bottom of the closure forms no part of the claimed design. The broken lines depict portions of the closure for a container neck that form no part of the claimed design.