Crossword Genius

Plain case has lost ivory top (6)

Ross

I believe the answer is:

patent

'plain' is the definition.
(I know that plain can be written as patent)

'case has lost ivory top' is the wordplay.
I cannot really see how this works, but
'top' could be 'pate' (pate is a kind of top) and 'pate' is present in the answer.
This explanation may well be incorrect...

(Other definitions for patent that I've seen before include "Licence for invention; obvious" , "Obvious - protection for inventors" , "Open the right to an invention" , "Licence for inventors; obvious" , "Like a medicine" .)

I've seen this clue in The Guardian.
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