A cluster chord is a chord that takes two or more of its tonalities sequencially from the scale pattern. In other words, the notes of the chord are grouped together in the scale. The Am7add9 that we were talking about is a cluster chord. The B and C are a half step apart in the C major scale, and that half step is what gives the chord the airy sound.

Take a look at this chord -


E |------------|
B |--3---------|
G |--0---------|
D |--4---------|
A |--0---------|
E |------------|


I'm not sure what to call it (Am7aug6sus4 maybe. LOL, I dunno), but it's definitely a cluster chord. A, Gb, and G right in a row. A-minor Dorian scale, by the way.

You will definitely know a cluster chord when you hear it. They always hit you right in the ear.