Strange about traitor following English mistake (7)
I believe the answer is:
erratum
'mistake' is the definition.
(I know that erratum is a type of mistake)
'strange about traitor following english' is the wordplay.
'strange' becomes 'rum' (similar in meaning).
'about' is an insertion indicator.
'traitor' becomes 'rat' (rat can mean a treacherous person).
'following' is a charade indicator (letters next to each other).
'english' becomes 'E' (abbreviation).
'rum' placed around 'rat' is 'rratum'.
'rratum' put after 'e' is 'ERRATUM'.
(Other definitions for erratum that I've seen before include "Printer's mistake" , "Typo" , "Printing mistake" , "Mistake by Latin scholar" , "Published correction" .)