Statement seems to be ignored unless something exec before it.

[SambaNeko]

This is needlessly convoluted:

var clientId = $("select#ClientId > option:selected").attr("value");
//equivalent to:
var clientId = $("#ClientId").val();

Can't really tell what the problem is; was unable to reproduce it with an assumptive setup. It might help if you can provide more context (the markup and any other related scripting).