Rogue’s about to steal one American diadem (5)
I believe the answer is:
tiara
'diadem' is the definition.
(I've seen this in another clue)
'rogue's about to steal one american' is the wordplay.
'rogue' becomes 'rat' (rat can mean a nasty person).
'about' shows that the letters should be reversed in order.
'to steal' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'rat' backwards is 'tar'.
'tar' placed around 'i' is 'tiar'.
'tiar'+'a'='TIARA'
(Other definitions for tiara that I've seen before include "it goes to one's head!" , "It bejewels" , "Jewelled head-ornament" , "Jewelled headwear" , "Type of diadem" .)