Crossword Genius

Special quality has unknown element (1,6)

Ross

I believe the answer is:

x factor

'special quality' is the definition.
(I've seen this before)

'unknown element' is the wordplay.
'unknown' becomes 'x' (algebra).
'element' becomes 'factor' (synonyms).
'x'+'factor'='X-FACTOR'

'has' acts as a link.

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

I've seen this clue in The Sun.
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