A stage concealed stones (6)
I believe the answer is:
agates
'stones' is the definition.
(I've seen this before)
'a stage concealed' is the wordplay.
'concealed' indicates anagramming the letters.
'stage' with letters rearranged gives 'gates'.
'a'+'gates'='AGATES'
(Other definitions for agates that I've seen before include "Hard coloured stones from a stage" , "Hard stones with concentric bands of colour" , "marbles" , "Ornamental stones" , "Pieces of impure quartz used as gemstones" .)