Right about a treacherous sort (3)
I believe the answer is:
rat
'treacherous sort' is the definition.
(a rat is a traitor)
'right about a' is the wordplay.
'right' becomes 'rt' (abbreviation as in Rt Hon- the Right Honourable).
'about' is an insertion indicator.
'rt' placed around 'a' is 'RAT'.
(Other definitions for rat that I've seen before include "one who betrayed" , "Despicable person" , "Rodent - deserter" , "Treacherous person" , "Cad" .)