The noun (Substantiv/Nomen) Fußball-bundesligisten is feminine in German. Therefore, the correct definite article in the nominative singular is die. So, you say: die Fußball-bundesligisten.
Bundesliga football clubs
Compound Words: The last word (base word) ALWAYS determines the gender. Most fundamental and reliable rule in German gender assignment This word is composed and its gender is always determined by the root word.
Die Vereine, die in der höchsten deutschen Fußballliga, der Bundesliga, spielen.
The clubs that play in the highest German football league, the Bundesliga.
Die Fußball-Bundesligisten kämpfen um die Meisterschaft.
The Bundesliga football clubs are fighting for the championship.