A scoundrel retreating, having snatched one item of jewellery (5)
I believe the answer is:
tiara
'item of jewellery' is the definition.
(I've seen this before)
'a scoundrel retreating having snatched one' is the wordplay.
'scoundrel' becomes 'rat' (I've seen this in other clues).
'retreating' says the letters should be written in reverse.
'having snatched' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'a'+'rat'='arat'
'arat' reversed gives 'tara'.
'tara' placed around 'i' is 'TIARA'.
(Other definitions for tiara that I've seen before include "it goes to one's head!" , "It bejewels" , "Ornamental head band" , "Item of jewellery" , "Small crown" .)