Crossword Genius

Fathead! Soon deduce a timeless quality, unfortunately, that's shared by Johnson and his cohorts (8,2,8)

Ross

I believe the answer is:

delusion of adequacy

I'm a little stuck... Click here to teach me more about this clue!

'fathead soon deduce a timeless' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'quality unfortunately that's shared by johnson and his cohorts' is the wordplay.
I cannot quite understand how this works, but
'quality' could be 'adequacy' (adequacy is a kind of quality) and 'adequacy' is located in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
This explanation may well be incorrect...

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