A traitor up to grab one small crown (5)
I believe the answer is:
'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).
'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" .)