Dissident Frenchman taking oath (8)
I believe the answer is:
renegade
'dissident' is the definition.
'frenchman taking oath' is the wordplay.
'frenchman' becomes 'rene' (common French name).
'taking' is an insertion indicator.
'oath' becomes 'egad' (I've seen this before).
'rene' going around 'egad' is 'RENEGADE'.
(Other definitions for renegade that I've seen before include "One who deserts a cause or group" , "Rebel, outlaw" , "Person who deserts from cause or faith" , "Turncoat, rebel" , "Perfidious" .)