Preferably inform on the lady (6)
I believe the answer is:
rather
'preferably' is the definition.
(I know that preferably can be written as rather)
'inform on the lady' is the wordplay.
'inform on' becomes 'rat' (I've seen this in another clue).
'the lady' becomes 'her' (her refers to a woman).
'rat'+'her'='RATHER'
(Other definitions for rather that I've seen before include "On the contrary or to some extent" , "On the contrary; more precisely" , "Pretty" , "Yes, please!" , "Slightly, more so than not (6" .)