The equivalent of a
single record / CD single for
audio cassettes, that is a cassette with just one song on it.
This
format is hard to find nowadays, even harder than prerecorded audio cassettes in general, which are on the decline anyway, due to
CDs,
MP3s and what not.
cassingle is not yet quite an official part of the English language (it's not in Merriam Webster's Dictionary, for example), but the term is used, especially in informal speech.