Turncoat Frenchman keeping old oath (8)
I believe the answer is:
renegade
'turncoat' is the definition.
('renegade' can be a synonym of 'turncoat')
'frenchman keeping old oath' is the wordplay.
'frenchman' becomes 'rene'.
'keeping' is an insertion indicator.
'old oath' becomes 'egad' (I've seen this before).
'rene' placed around 'egad' is 'RENEGADE'.
(Other definitions for renegade that I've seen before include "Traitor" , "A deserter from a cause or faith" , "Dare Gene to become a rebel" , "One who deserts cause or faith" , "Treacherous person" .)