Something valuable in one's house (6)
I believe the answer is:
gemini
'house' is the definition.
(each star sign has an astrological house)
'something valuable in one's' is the wordplay.
'something valuable' becomes 'gem'.
'one' becomes 'i' (Roman numeral).
'gem'+'in'+'i'='GEMINI'
(Other definitions for gemini that I've seen before include "Heavenly sign of the Twins" , "space programme" , "lot of stars" , "star group" , "Twins constellation" .)