At some stage writing on stones (4)
I believe the answer is:
gems
'stones' is the definition.
(precious stones)
'at some stage writing on' is the wordplay.
'at some stage' becomes 'ge' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'writing' becomes 'ms' (abbreviation for manuscript).
'on' is a charade indicator (letters next to each other).
'ge'+'ms'='GEMS'
Can you help me to learn more?
(Other definitions for gems that I've seen before include "Precious stones" , "Valuable stones" , "Jewels" .)