What does "クズな" mean?


Original sentence: 審判がクズなだけ！ Google translates it as "Referee just scrap!" So
if we analyze the sentence:

[code]

    審判(referee) が(GA particle) クズな(???) だけ(only?)
    
[/code]

What part of speech is クズな and what does the sentence really mean?




クズ(屑) literally means ゴミ, waste, trash, litter, garbage, rubbish... and it's
often used like "crap" or "a piece of crap". I think it means something like
[役立]{やくた}たず or [無能]{むのう} (worthless, good-for-nothing..?).  
  
審判 The referee  
が the case particle as a subject marker  
クズ crap, rubbish  
な The auxiliary verb だ  
だけ！only, It's only~~  
  
So I think it's like... "It's only he's a crap referee!" "It's only because
the referee is rubbish!"




クズ(屑) literally means ゴミ, waste, trash, litter, garbage, rubbish... and it's
often used like "crap" or "a piece of crap". I think it means something like
[役立]{やくた}たず or [無能]{むのう} (worthless, good-for-nothing..?).  
  
審判 The referee  
が the case particle as a subject marker  
クズ crap, rubbish  
な The auxiliary verb だ  
だけ！only, It's only~~  
  
So I think it's like... "It's only he's a crap referee!" "It's only because
the referee is rubbish!"

