Special quality (1,6)
I believe the answer is:
'special quality' is the definition.
(X factor can mean star quality)
This is all the clue.
(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" .)