Certainly it's the smart thing to do (6)
I believe the answer is:
indeed
'certainly' is the definition.
(I've seen this before)
'the smart thing to do' is the wordplay.
I cannot really understand how this works, but
'smart' could be 'in' (as in fashionable) and 'in' is present in the answer.
The remaining letters 'deed' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'it's' is the link.
Can you help me to learn more?
(Other definitions for indeed that I've seen before include "Absolutely" , "Truly or admittedly" , "In truth, in fact" , "still" , "To be sure" .)