Fashionable woman seen with duke? Quite so (6)
I believe the answer is:
indeed
'quite so' is the definition.
(both can be used to express agreement)
'fashionable woman seen with duke?' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'woman' becomes 'Dee' (woman's name).
'seen with' says to put letters next to each other.
'duke?' becomes 'd' (abbreviation).
'in'+'dee'+'d'='INDEED'
(Other definitions for indeed that I've seen before include "In truth, in fact" , "Yes, truthfully" , "beyond doubt" , "Without question" , "Really, without doubt" .)