Bowled over a scoundrel snatching one item of jewellery (5)
I believe the answer is:
tiara
'item of jewellery' is the definition.
(I've seen this before)
'bowled over a scoundrel snatching one' is the wordplay.
'bowled over' says the letters should be written backwards.
'scoundrel' becomes 'rat' (rat can mean a nasty person).
'snatching' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'a'+'rat'='arat'
'arat' placed around 'i' is 'arait'.
'arait' in reverse letter order is 'TIARA'.
(Other definitions for tiara that I've seen before include "Jewelled coronet" , "High diadem" , "Pope's triple crown" , "Jewelled headband" , "Item of jewellery" .)