A traitor up to grab one small crown (5)
I believe the answer is:
tiara
'small crown' is the definition.
(I've seen this in another clue)
'a traitor up to grab one' is the wordplay.
'traitor' becomes 'rat' (I've seen this before).
'up' is a reversal indicator (in down clue: letters go upwards).
'to grab' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'a'+'rat'='arat'
'arat' reversed gives 'tara'.
'tara' enclosing 'i' is 'TIARA'.
(Other definitions for tiara that I've seen before include "Women's head ornament" , "Item of jewellery" , "Ornamental headpiece" , "Pope's triple crown" , "Jewelled crescent-shaped headdress" .)