Special quality (1,6)
I believe the answer is:
x factor
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">special quality</span>' is the definition.<br/ ><span class="explanation-format__explanation">(X factor can mean star quality)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for x factor that I've seen before include "Its judges hear appalling submissions" , "Magical 'something'" , "Noteworthy special quality" , "'Hard to define but important special property (1,6)'" , "Popular TV talent show" .)