Hold forth in Old English about traitor (5)
I believe the answer is:
orate
'hold forth' is the definition.
(I've seen this before)
'old english about traitor' is the wordplay.
'old english' becomes 'OE'.
'about' means one lot of letters goes inside another.
'traitor' becomes 'rat' (rat can mean a treacherous person).
'oe' going around 'rat' is 'ORATE'.
'in' is the link.
(Other definitions for orate that I've seen before include "talk at length" , "Make a lengthy speech" , "Hold forth" , "Speak in public" , "Speak pompously and at length" .)