It's a compound noun in form that functions as an adjective. It modifies the noun boom.
Before the superclub boom that happened either side of the millennium, DJs started out as relatively anonymous record changers, with a handful known only to committed dance acolytes.
 You know it's an adjective if you delete it and resulting sentence remains grammatical: You know it's an adjective if it answers the question What kind of?:
- Before the superclub boom ...
- Before the boom ... <deleted superclub> You know it's a noun if it functions as the subject or object of the verb:
- What kind of boom? A superclub one.
- Before the superclub that happened ...
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