Crown, one stolen by a knave, turned over (5)
I believe the answer is:
tiara
'crown' is the definition.
(thesaurus)
'one stolen by a knave turned over' is the wordplay.
'one' becomes 'i' (Roman numeral).
'stolen by' means one lot of letters goes inside another.
'knave' becomes 'rat'.
'turned over' says the letters should be written backwards.
'a'+'rat'='arat'
'i' inserted inside 'arat' is 'arait'.
'arait' backwards is 'TIARA'.
(Other definitions for tiara that I've seen before include "Type of diadem" , "Jewelled head-ornament" , "Decorative headgear" , "Jewelled headband" , "Jewelled coronet" .)