A ‘tail only’ is a version of a jingle with sung vocals heard in the last few seconds only. This is similar to a top and tail, but there is no ‘top’. A tail only jingle is used when you want to fit more voice over content in the advertisement but you want part of the jingle to be heard at the end, for greater effect. Tail only jingles are used often in 15 second ad slots. Because the allotted time is quite small, there is only enough room for a voice over at the start and the ‘tail’ at the end (usually the business name being sung).
For more definitions, read the post jingle lingo, what does it all mean.