In addition are real, cut, gems (6)
I believe the answer is:
pearls
'gems' is the definition.
(pearl is a kind of gem)
'in addition are real cut' is the wordplay.
'in' indicates putting letters inside.
'addition' becomes 'ps' (**).
'cut' indicates anagramming the letters.
'real' anagrammed gives 'earl'.
'ps' enclosing 'earl' is 'PEARLS'.
(Other definitions for pearls that I've seen before include "Jewels from bi-valves" , "Oyster jewels" , "They're found in 9dn" , "Stones" , "Don't cast these jewels before swine" .)