This is a word which has been added to the Oxford English Dictionary in its June quarterly update. The OED says of it:
One surprise of this range was the fecundity of riff, n (and riff, v) in producing new nouns referring to the playing of catchy musical phrases. Besides riffage, this update also includes new entries for the whimsical riffola, n. and the retro rifferama, n. These words entered the English language amid an explosion of popular music journalism in the second half of the twentieth century, coined by critics who apparently felt limited by the staid predictability of riffing, n. The three new entries are only the tip of a neologistic iceberg: OED's files also contain examples of riffery, riffdom, riffmongery, and riffology, among others which may eventually be considered for inclusion in future updates.
I would never have thought of harmless little riff in terms of "the tip of a neologistic iceberg", and I daresay that neither would the great players of them (Buster Bailey, clarinet, for instance). I must let the editors know that in listing other derivatives for possible future inclusion they have failed to mention many important ones such as: rifflike, rifferoo, riffmanship, and of course the ever-popular if slightly vulgar phrase riff off, all of which have appeared in print and therefore qualify for an entry.