Seriously old walls in home (9)
I believe the answer is:
earnestly
'seriously' is the definition.
(I know that seriously can be written as earnestly)
'old walls in home' is the wordplay.
'old' becomes 'early' (similar in meaning).
'walls in' indicates putting letters inside (I've seen this in other clues).
'home' becomes 'nest' (a bird's home).
'early' placed around 'nest' is 'EARNESTLY'.
(Other definitions for earnestly that I've seen before include "Showing commitment" , "Sincerely and seriously" , "Showing serious intent" , "In all seriousness" , "Zealously" .)