Deserter, that woman? On the contrary (6)
I believe the answer is:
rather
'on the contrary' is the definition.
('rather' can precede a statement to the contrary)
'deserter that woman?' is the wordplay.
'deserter' becomes 'rat' (both can mean a traitor).
'that woman?' becomes 'her' (her refers to a woman).
'rat'+'her'='RATHER'
(Other definitions for rather that I've seen before include "A bit" , "On the contrary; sooner" , "On the contrary or to some extent" , "somewhat > sooner? > Quite" , "To a limited degree" .)