Plain case has lost ivory top (6)
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" .)