A stage built of stones (6)
I believe the answer is:
agates
'stones' is the definition.
(I've seen this before)
'a stage built' is the wordplay.
'built' is an anagram indicator.
'stage' with letters rearranged gives 'gates'.
'a'+'gates'='AGATES'
'of' acts as a link.
(Other definitions for agates that I've seen before include "Ste aga up for the stones" , "Hard stones with concentric bands of colour" , "Hard coloured stones from a stage" , "Multi-coloured quartzes" , "Ornamental stones" .)