Is there a word that describes a slang term that was, itself, derived from (or riffs on) another slang term? I was under the impression that the term Snowclone described ths phenomenon, but it seems to be about something else entirely.
Slang term coinage may describe what you are referring to:
A new word or phrase.
The invention of new words.
I think you may be also referring to : slang semantic change.
Generally, slang terms undergo the same processes of semantic change that words in the regular lexicon do.
Eric Partridge, cited as the first to report on the phenomenon of slang in a systematic and linguistic way, postulated that a term would likely be in circulation for a decade before it would be written down.Nevertheless, it seems that slang generally forms via deviation from a standard form. This "spawning" of slang occurs in much the same way that any general semantic change might occur. The difference here is that the slang term's new meaning takes on a specific social significance having to do with the group the term indexes.
Still, while a great deal of slang takes off, even becoming accepted into the standard lexicon, much slang dies out, sometimes only referencing a group. An example of this is the term "groovy" which is a relic of 1960's and 70's American "hippy" slang. Nevertheless, for a slang term to become a slang term, people must use it, at some point in time, as a way to flout standard language. Additionally, slang terms may be borrowed between groups, such as the term "gig" which was originally coined by jazz musicians in the 1930s and then borrowed into the same hippy slang of the 1960s.