The fashionable thing to do? Certainly (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'the fashionable thing to do?' is the wordplay.
'the fashionable' becomes 'in' (in can mean fashionable or trendy).
'thing to do?' becomes 'deed' (a deed is an action).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Truthfully, really" , "Really - truly" , "Really - what a surprise!" , "Absolutely" , "Certainly, quite so" .)