Stone Age circles advanced over time (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'age circles advanced over time' is the wordplay.
'circles' indicates putting letters inside.
'advanced' becomes 'a' (abbreviation).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'time' becomes 't' (abbreviation).
'a'+'t'='at'
'age' enclosing 'at' is 'AGATE'.
(Other definitions for agate that I've seen before include "marble" , "Striped stone" , "Quartz with bands of colour, used in jewellery" , "Stone with coloured bands" , "Old drama critic" .)