Variable not defined when using YouTube API?

The $.get() method like $.ajax() or $.json() are asynchronous methods that means you receive the data AFTER your code has executed return TTSearchResultID

Here is one way of doing what you want to achieve (I also removed your useless $.each because you were requesting only ONE item) :

function QueryGetta0() {
            var TTsearchQuery = 'lana del rey';
            return TTsearchQuery;
}


function QueryGetta1() {
    return $.get(
                "https://www.googleapis.com/youtube/v3/search",{
                    part: 'snippet',
                    maxResults: 1,
                    q: QueryGetta0(),
                    type: 'video',
                    key: 'AIzaSyCvk3NNMQASZgFkCNxIp9jH-l8O0PXhDUo'}
            );  
}



$('#clicker').on('click', function() {        
        QueryGetta1()
        .done(function(data) { //Inside this function, do everything you want with data
            var TsearchResultID = data.items[0].id.videoId;
            alert(TsearchResultID);
        });
});

Fiddle : http://jsfiddle.net/p5y4t9co/6/

Explanation :

$.get returns a jqXHR object. On this object we can use .done(pCallbackOnSuccess) or .fail(pCallbackOnFail).

The .done method is called when the request is successful, the callback will receive the requested data as a parameter and if the request encoutered an error, it's .fail which is called, the callback will receive the error object.

Exemple of using both :

    $.get(options)
        .done(function(data) {
            //do what you want with data
        })
        .fail(function(error) {
            //do what you want with the error
        });