Special, undefinable quality (1,6)

I believe the answer is:

x factor

(Other definitions for x factor that I've seen before include "Its judges hear appalling submissions" , "Noteworthy special quality" , "Magical 'something'" , "'Hard to define but important special property (1,6)'" , "Noteworthy special talent" .)

I've seen this clue in the Sydney Morning Herald.

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