Certainly at home with something to be done (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'home with something to be done' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'with' is a charade indicator (letters next to each other).
'something to be done' becomes 'deed' (a deed is an action).
'in'+'deed'='INDEED'
'at' acts as a link.
(Other definitions for indeed that I've seen before include "Truthfully, really" , "Absolutely" , "Very much so" , "Word expressing surprise, disbelief, etc" , "In fact, really" .)