A stage concealed stones (6)
I believe the answer is:
Here is my best explanation:
'stones' is the definition.
(I've seen this before)
'a stage concealed' is the subsidiary indication.
'concealed' indicates anagramming the letters.
'stage' with letters rearranged gives 'gates'.
(Other definitions for agates that I've seen before include "Banded semiprecious stones", "Hard coloured stones from a stage", "Ornamental stones", "marbles", "Pieces of impure quartz used as gemstones", "Gemstones", "Stones to be found in a stage", "Hard stones with concentric bands of colour", "[RING]-fillers required" and "Ste aga up for the stones".)