In retirement Duchess appears somewhat old-fashioned (5)
I believe the answer is:
passe
'old-fashioned' is the definition.
(I know that old-fashioned can be written as passe)
'in retirement duchess appears somewhat' is the wordplay.
'in retirement' is a reversal indicator (retire can mean to go back or backwards).
'somewhat' indicates a hidden word.
'essap' is hidden within 'duchess appears'.
'essap' backwards is 'PASSE'.
(Other definitions for passe that I've seen before include "No longer fashionable" , "-- -partout" , "Master key - picture mounting" , "Past its prime - in France" , "Old hat" .)