Crown — gold one is used in court (7)
I believe the answer is:
coronet
'crown' is the definition.
(I know that coronet is a type of crown)
'gold one is used in court' is the wordplay.
'gold' becomes 'or' ('or' is the heraldic name for gold).
'is used in' means one lot of letters goes inside another.
'court' becomes 'ct' (abbreviation used in road names).
'or'+'one'='orone'
'orone' put into 'ct' is 'CORONET'.
(Other definitions for coronet that I've seen before include "Diadem" , "Simple crown" , "jewelled headdress" , "Sub-lieutenant" , "in the band" .)