Ultimately search each large building (4)
I believe the answer is:
hall
'large building' is the definition.
(hall can mean a mansion or stately home)
'ultimately search each' is the wordplay.
'ultimately' suggests the final letters.
'each' becomes 'all' (similar in meaning).
The final letter of 'search' is 'h'.
'h'+'all'='HALL'
(Other definitions for hall that I've seen before include "Large meeting room" , "Vestibule, lobby" , "Auditorium" , "Chamber" , "Toad ___ (Toad's home)" .)