Unquestionably, it's the smart thing to do (6)
I believe the answer is:
indeed
'unquestionably' is the definition.
(I've seen this before)
'the smart thing to do' is the wordplay.
'the smart' becomes 'in' (as in fashionable).
'thing to do' becomes 'deed' (a deed is an action).
'in'+'deed'='INDEED'
'it's' is the link.
(Other definitions for indeed that I've seen before include "Very much so" , "Certainly, quite so" , "Truthfully, really" , "Really - truly" , "Yes, truthfully" .)