The working code is here, in your code nothing is wrong please check what txtContent render at client side. Also check jQuery is loaded properly.
<div>
<asp:TextBox ID="txtcontent" runat="server" class="form-control" TextMode="MultiLine"></asp:TextBox>
<br />
<div onclick="resetproduct()">Click to Cancel</div>
</div>
<script>
function resetproduct() {
$("#<%= txtcontent.ClientID %>").val("");
return false;
}
</script>