Stone in one sign
I believe the answer is:
gemini
'sign' is the definition.
(I know that Gemini is an sign)
'stone in one' is the wordplay.
'stone' becomes 'gem' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'gem'+'in'+'i'='GEMINI'
(Other definitions for gemini that I've seen before include "Third zodiacal sign" , "Twins star sign" , "set of twinklers" , "space programme" , "House" .)