Turncoat in retreat defied a general? Not completely (8)
I believe the answer is:
renegade
'turncoat' is the definition.
(I know that turncoat can be written as renegade)
'retreat defied a general? not completely' is the wordplay.
'retreat' is a reversal indicator.
'not completely' says the answer is hidden in the clue.
'edagener' is hidden within 'defied a general'.
'edagener' reversed gives 'RENEGADE'.
'in' is the link.
(Other definitions for renegade that I've seen before include "Turncoat, rebel" , "Traitor" , "Treacherous person" , "Treacherous deserter" , "not someone true to the cause" .)